RU & CIS stand for Russia and Commonwealth of Independent States (i.e. Ukraine) respectively.
These are the regions where I am able to get most games cheaper from. Most games have the same steam-id as their US counterparts, making them completely region-free. Certain games, however, have different steam-ids in the RU/CIS regions. Whenever this is the case, you need to check the
steam database to find out if the game is region-locked or not.
If a game is tagged with "RU+CIS," it generally means that there is a chance the game is region-locked, or only has the Russian language available (depending on what is shown in its steam database entry). It is also a possibility that there are no restrictions on the game.