Does anybody know what Project Settings > Modules > [module name] > Source Roots does?

Huh?
In that very dialog shown in your screenshot, click on the "Help" button to find this:

"Select the check box next to a root to have the classes from this root included in the corresponding artifact."

Oh, if you are on a Mac, then the "Help" button is probably the little "?" button in the lower left.
(On my unix machine it is a button with the text "Help" besides the other regular buttons.)

Thanks, it's barely visible on my machine. Good to know it's there now.