When you walk around the map, you may notice green leaves appear on the map. You may find green leaves in any places: parks and by the water. They appear randomly do not stay at same point longer.
What green leaves mean
Green leaves indicate a point where you can find wild Pokemon. That means if you see many points with green leaves on the map, the possibility of finding wild Pokemon is higher.
You can find and capture Pokemon at a point where there are no green leaves. Additionally, it is not necessary to see Pokemon at points with green leaves. You may see nothing once you walk to green leaves.