If you want to see the actual activity on the instance, click through the "See What's Happening" link which takes you to the public view:
<https://toot.thoughtworks.com/public>
Note that a specific profile's public URL page will be, e.g.,
https://toot.thoughtworks.com/@mfowler
<https://toot.thoughtworks.com/@mfowler>
Yes, there are a few concepts to understand. Some six millions of profiles worth of people seem to have worked it out to date.
This is a world in which people are familiar with webmail (email access through an HTTP gateway), and of both Web- and App-based interfaces to services such as Facebook, Twitter, Instagram, TikTok, SnapChat, and Reddit.
I'm reasonably comfortable in the thought that whether or not the Great Unwashed Masses think in terms of "HTTP Gateway" or Browser vs. App, they'll work through this particular Sysiphian obstacle without a great deal of effort.
As a nontechnical user, I am immediately confused and closed the page without learning anything. The idea that a company could "reach millions of users" with Mastodon is ridiculous.
I understand the federated social web has a technobabble aspect to it. I am sorry you aren't with it but that's honestly not my problem.