The rustling grass is driving me crazy - does it actually mean something? I've tried walking towards it, I mean I have actually walked in circles for five minutes in a Home Depot parking lot and in my neighborhood, but nothing ever comes of it. I've tried tapping the screen where the grass is rustling - nothing. Sometimes I see little pieces of green, other times it is substantially bigger. But never a pay off. What am I missing?
Grass Shaking: A Pokemon may or may not reside there. Higher chances of encountering a wild Pokemon. -- Source
When you are tracking a Pokemon by how close they are, you might want to try to go to the grass spots first. You may be lucky and the pokemon is there. If it is not there you just continue looking for it, but it was still worth trying since the spots have higher probability of having a pokemon nearby.