The Grass temple is the first temple you go to in the game.
In the Grass temple you first see an example of movable blocks in simple puzzles.
Also in the Grass temple you get your first item. The Boomerang.
There is a locked red door here in which you can only open by buying a key of the corresponding color (which in this case is red) for 1000 gol at Jim's Shack (or by finding one in a temple). Unless, that is, you have the Super Key, which opens them all and eliminates the need. Inside the room behind this door lies a life shard.
The boss in the Grass temple is a Giant Snake called Eelagon.
Once you beat Eelagon you rescue ie sage Indy and get the yellow orb.
See also: Walkthrough