Grats is Short for Congratulations.
Grats is something you say to give someone props, commendation, lovin', and basically some variant of "good job" for doing something well. Congrats, you now know what grats means.
• Ay yo man, grats on getting the new car.
• Just wanted to throw some grats your way regarding the new baby.
• Grats on being the new Pope, nerd.