I'm not familiar with it either, but this seems to provide a description https://en.wikipedia.org/wiki/Payment_for_order_flow

Seeing/selling order flow is a term of art in finance. Seeing order flow refers to the ability to trade against customer orders. Selling order flow is getting paid by someone else for the ability to trade against customer orders.

Imagine that a stock is bid at $100.00 and offered at $100.01. Assume that market makers estimate the fair price to be $100.005 unconditionally.

A customer sends a marketable buy limit order at $100.01.

If this hits the exchange then the person who's offering at $100.01 will make $0.005.

If the order flow gets sold (i.e. someone gets to see the flow before it hits the exchange) then the internalizer can fill it at $100.01 and make the $0.005 themselves instead of letting someone else on the exchange do it. It has nothing to do with front running or even information.

This is really valuable because you're not competing for speed with other market participants and you expect the customer flow to be uninformed so the trade is less likely to move against you before you trade out.

This is a problem for market structure because it discourages people from quoting on the exchange.

User data on what stocks they buy and sell

Except between the time the user clicks buy and when it the buy actually happens. Basically hedge funds are able to receive and parse this data, and change their orders to an advantage before the users order hits the actual exchange

Its analogous to a MITM attack

Despite my wording its not necessarily bad, but I think users should have a better understanding of it.