Central Park, besides being a world-class urban resource, is a showcase of New York City geology. The beautifully exposed outcrops of schist and gneiss bear traces of the ice ages, when continental glaciers scraped their way across the region leaving grooves and polish on the tough bedrock. When the glaciers melted, they dropped whatever they were carrying, including some large boulders like this. It has a different composition from the ground it sits on and clearly comes from elsewhere.
Glacial erratics are only one kind of precariously balanced rocks: those also occur under other circumstances, especially in desert settings (here's more on how those arise). In some areas they are even useful as indicators of earthquakes, or their long-term absence.
For other views of Central Park, see the walking tour of trees in Central Park North and South by Forestry Guide Steve Nix or the Central Park Movie Locations by New York City Travel Guide Heather Cross.
