The word "grats" or gratz is a truncated form of the word congratulations, or congratz. The word is used by people in online games (mainly mmorpg’s and mostly it is seen in PristonTale) its use is to congratulate someone on accomplishing something. I.e. moving up a level.
(Gold level up! appears above Jacl3's head)
Jacl3: w00t level 34!
l337357: grats man
A truncated form of congratulations as stated; however it is a hollow form. There is little emotion, you don't care you are envious or it doesn't matter. You join the bandwagon of everyone else saying it.
The word is often used as just "grats" by itself. When it is used in this form is is surely a carefree statement.
Grats on leveling
oh hey bob grats on getting your mount.