The name says it al. How to get it?
Import Unity’s Terrain Package from the asset tab. Which I believe provides simple grass textures which you can press and click where you want grass. You need to mess around in the terrain inspector.
Also, I google searched it and found this as the first link Unity - Manual: Grass and other details
Try google before you post.
First and foremost, learn how to use the terrain engine. Here is a good tutorial on using grass specifically:
Check out the rest of the videos in that series to learn how to make good terrain overall.