Hi Văn Tuấn Kiệt,
잔디 = grass. lawn, turf
풀 = grass
잔디 refers to short grass and usually that is mowed regularly.
풀 refers to grass in general. It includes all kinds of grass regardless of whether it is mowed or not.
Ex.
- 우리는 공원 잔디 위에 앉아있었다.
(We were sitting on grass in park)
- 아기 염소는 풀을 먹고 있었다.
(The baby goat was eating grass)
Hope it helps,