If you're a gamer you're probably aware of Valve's latest move with Steam; making Workshop mods now have the option for the developer to lock them behind a paywall (of which 75% goes to Valve and the publisher while the remaining 25% goes to the mod developer. Nothing dodgy there).
This has caused all sorts of completely foreseeable issues and has many calling foul. What about a simple "donate" option instead? Of course not! That would make too much sense.
If you're not a gamer or not familiar with "mods" (which is short for player created modifications) they are basically any sort of modification to an existing game by non-professionals - usually the gaming community and players themselves - and has a long history dating all the way back to Wolfenstein 3D (perhaps earlier?).
They have always been free, done by the gamers for the gamers, purely for the love of it. Valve and Steam no longer want that. They just see the potential $$ they are missing out on.