When Manhattan’s grid system was planned in 1811, surveyors started driving spikes into the places where intersections would soon be created. This original plan did not include Central Park, which wasn’t approved of and built until the 1850s.
This spike is one of the only original spikes still around from the 1811 plan. It represents the planned location of 65th Street and 6th Avenue, which was never built because Central Park took over that part of Manhattan.
More information about the original grid plan and John Randel, Jr. can be found in the profile the Times wrote in 2011.