While wandering around in Pokémon GO, you'll occasionally see what appears to be leaves fluttering around nearby. This is actually meant to be Pokémon "rustling in the grass," but whatever the intention, it means that there may be a wild Pokémon in that area.
While there's no use in tapping on the leaves (it does nothing), they can be helpful in your search for Pokémon to catch.
Try using the leaves in combination with the "nearby" list to find Pokémon near you. In the nearby list, you can see how many "footprints" away any given Pokémon is. Use the green rustling leaves to point yourself in the right direction. If there is indeed a Pokémon there, you will see it appear on your screen.
Tap the Pokémon to open the capture screen, huck a Poké ball its way, and hope you're accurate enough to snatch it up.
So, if you see leaves, get yourself over to them ASAP. And if you're lucky, you'll find something a whole lot better than a stupid Weedle.