Unity is a game engine and a bunch of ancillary services, analytics and tracking and what not. It’s been free to use and publish games with as long as your company revenue was under a certain amount. Over that amount and you’d have to buy a license for I think about $1600 a year.
The brouhaha was because they changed their income model to charge people/companies who create their game using the unity engine to make games on a per install basis. Up to 20cents per install of your game ( but only if your revenue was over $200k AND installs was over 200k, raising to $1m AND 1m installs with the unity pro license) .
The changes would take place next January leaving developers with very little time to make any changes to their revenue model. Unity (the company) also changed the terms of use of Unity (the game engine software) so that it was retroactive across all previous versions of unity, ie. If you didn’t like the new terms you couldn’t just carry on using an older version of it.
If you were being charitable you’d call it a clumsy launch or even ill considered. But it went down like a bucket of cold sick with the game dev’ community who viewed it like a greedy shakedown.