The Renown currency is what you earn for just about everything you do in Rainbow Six Siege, namely completing Situations and Terrorist Hunt or competing in Online Multiplayer matches. You can use Renown to unlock Original and Post-Launch Operators, weapons, skins, and more.
R6 Credits are an optional currency that you can purchase. R6 Credits can be used to buy Renown Boosters, which allow you to generate Renown faster while playing. The R6 credits can also be used to unlock new Operators and some exclusive premium skins.