1. You are required to pay a certain amount (either percentage or flat fee, I'm not sure) of your paycheque into a new political organization, the union
2. You are entitled to periodically vote on the who to elect to leadership of the union
3. The leadership of the union represents the entirety of the staff at the company in any and all negotiations and disputes with management. The leadership of the union will engage in collective bargaining with management, establishing a single working contract that encompasses every employee<->employer relationship in the company.
So, right now, in a non-unionized company, if you go to work there, you and the company negotiate a contract which is the terms of your working relationship. Frequently, the company will dictate the terms of the contract and then you will exercise some marginal negotiation power regarding, say, pay or vacation time or whatever, although (especially for specialists and highly paid staff) these are far more open to negotiation, generally, than people think.
When you do this, this is a private relationship between you and your employer. Your coworkers are not involved. They have their own private individual relationships with the employer that have nothing to do with yours. Their contracts could be radically different from yours. That's between them and the company.
What a union does is get in between you and the employer, and collectivizes all of those individual negotiations into one larger, general negotiation. So now, instead of you negotiating a private contract with the employer, the union will negotiate a general contract that applies to _all_ staff, and then you will sign that contract.
The main argument in favour of unions like this is that by collectivizing the bargaining, it gives them negotiation power. So imagine, for instance, that you are getting a job at Google. As an individual, if you don't like the contract Google is giving you, you can demand dramatic changes to it. And then Google will laugh at you, spit in your face, and kick your ass out the door, because they're one of the richest corporations on the planet and you dare to think that _you_ can dictate terms to _them_? But, if the staff at Google were unionized, suddenly it's different. Suddenly, when the union pushes back on Google and demands drastic changes, Google has to negotiate in good faith. This is because the union represents _all_ of the employees; if the union threatens to walk away from the negotiation, then _all_ of Googles employees stop working. Google might be able to kick any individual out the door, but if the entirety of Google's staff stops working suddenly there are massive problems.
So why might you not want a union? Well, there's a few reasons. One might be obvious from the previous paragraph. Say you are quite happy with your current job at Google, but some of the unions _other_ members are unhappy and are demanding change. Eventually, negotiations come to an impasse. The union threatens to walk away. If they do that, you _must_ stop working. The fact that you are quite happy with your job doesn't matter. You are bound by their collective agreement, and they just suspended their collective agreement.
More generally, unions take a distributed, private, individual process and turn it into a politicized, public, collective process. This can reduce the company's flexibility to make different arrangements with different groups of people, and if you are someone who benefits from such a different arrangement you might rationally be opposed to this. Additionally, if you are someone who is heavily skeptical of the ability of political processes to make effective decisions, you might not want a board of elected union activists making decisions on your behalf.
Unions also cost money. I admit I don't know how much they cost; for all I know it's $5 per paycheque. But fees can be substantial; a long time ago, back when I was a clerk at Safeway, my salary was $7.50/hr and my union dues were $1.25/hr. This was almost 20% of my (pre-tax!) pay. If you feel that the benefits you get from collective bargaining do not outweigh your unions dues, you might start to wish you were not a member of the union
(Note that, generally, when your workplace votes to unionize, you are _required_ to pay those dues, regardless of whether or not you wish to become a member of the union. In some states, Republicans have succeeded in passing what are known as "Right-to-work laws", which prohibit unions from collecting fees involuntarily from non-members. However, California has no such laws).
Another reason why you might be opposed to unions, which might sound absurd at first glance, is that by fighting for the best interests of all of their members, they may sacrifice the best interests of some of their members. Because most unions are national-scale, with various chapters for different companies, your local union chapter may occasionally require you to do things that sacrifice your own best interest in order to support a remote chapter (striking in solidarity is one example: The union calls a nationwide strike even though there's nothing wrong with your particular office). A union might decide to make strategic sacrifices. Perhaps a union is in dispute with an employer that has multiple offices. A union might negotiate very aggressively in one office, overplaying their hands and ultimately losing substantially for the workers in that office, in order to credibly signal their resolve for when they are negotiating in the second office. Even though this is better overall, you probably don't think so if you're laid off because you were unlucky enough to work in office #1.
Finally, unions can cripple an industry with excessive demands. Unions may gain concessions from employers that are economically unsustainable. Eventually, this can cause the company to go out of business as more nimble competitors are able to perform better in the market than the unionized company is. In some dramatic cases, the company may decide that dealing with the union is too much trouble, and shut down operations in a given area entirely, putting all of the members out of a job.
A very real fear that I have is that I observe that a lot of the success of Silicon Valley is driven by a culture of work. Some might say _over_-work. Maybe this is not healthy for the people doing it, but it certainly seems that a key component of many tech companies' success is people working weekends and evenings when the situation demands it. One likely outcome of tech unionization, if I was to guess, is unions collectively enforcing a strict 40-hour work week, no exceptions. This sounds great, overworking is a scourge on all of us. It sounds great, right up to the point where a bunch of un-unionized starts who are burning the midnight oil start offering dramatically better products than the unionized established company, who can't compete because they can't leverage as much manpower towards their projects due to contractual limits on working hours.
