You can tag games from the tag tab of the information panel on the game page.
To add tags to a game, simply type them in the box; you will be able to select previously entered tags.
To find tagged games, go to the Game Database page (Chess > Search Game Database), and select the tag you are interested in from the dropdown list.
Tags can be used for any purpose which require gathering the games into groups. Here are some of the ways that SchemingMind members are taking advantage of the game folders feature (of course folder names are given as example, you can pick any names you like).
Watched Games - noticed interesting games just played by other players (one can save every interesting game to such folder, then visit it to view how are they progressing).
My Racing Kings, My CrazyHouse, etc. - currently played games in different variants.
My Good Games - the place where one can save particularly good own (finished) games.
Instructive: Najdorf, Instructive: Kings Gambit (usually the openings one is frequently playing) - the place where noticed instructive games (both own, and played by others) can be saved for easy future reference.
My: Najdorf, My: French (as above, openings from own repertoire) - completed games grouped by opening, so one can quickly review what moves worked or didn't work in a particular opening. This is handy not only for standard chess, but also for certain variants in which exact opening play is important.
Instructive variant games (separate tag for every frequently played variant).
And, possibly, more...