Proton is basically Wine bundled with other software, like DXVK and VKD3D, to run windows games.
You have to enable it in the Configuration window inside of Steam if you haven't done that yet. Enabling it is all you have to do and it will be used automatically.
Yep. You can have a look at ProtonDB to have an ideia of how well a game runs through Proton.
It's not completely correct as some games marked with lower ratings will work flawlessly, and some with higher ratings will probably give you some trouble, but it's a really useful resource.