Dating back to the days when horses filled the streets of the city, boot scrapers were a common tool used to clean off the bottoms of one's shoes before entering a house. There are many remaining in NYC, but it's rare to find one out here in Greenpoint.
Aha! It's part of the MillionTreesNYC program. The flip side of the tag has care instructions on it. The trees in the background are also part of this program, and are also Canada red chokecherries.