The Steam Workshop is a mod repository for Steam Games. When a developer releases a game on Steam, and that game has mod support, they have the option to tie it into the Steam Workshop. ... You can also view a list of every game that supports the feature. You can access the Steam Workshop directly from your Steam library.

Is Steam Workshop safe?

They're safe. Rimworld, Caves of Qud, Prison Architect, etc are not. They allow modders to use a non-sandboxed nor restricted C# enviroment, which can be as malicious as any anything else you run on your computer. Those games even inform you about this.

How do Steam Workshop mods work?

Open the Game Launcher -> Mods window to see the list of available mods. Available in the sense that they're downloaded and copied into the InstallationFolder\Mods. To activate a mod, simply tick the little checkbox on the left of a mod name in the list and click Apply.