[click on pictures to enlarge - taken via iPhone]
No need to scatter bread crumbs along your walk to find your way back home.
Every lamppost in the park has a number printed on it. The numbers refer to the cross street you are near, and how far into the park you are, in terms of the number of posts from the street. In addition to the cross street, some of them also bear a W, C, or E, corresponding, respectively, to the western, central or eastern part of the park.
So, the lamppost above is located where East 86th Street would be. And it tells you that you are 2 posts in from 5th Avenue.