The New York City Subway map is a mix of symbols that are intended to help riders navigate the system. The New York Subway is a large and complex system that takes a bit of time for those of us who did not grow up in New York to figure out. The map covers 468 stations and 660 miles of track. For comparison the Bay Area Rapid Transit System, the system I rode growing up, has 44 stations and 104 miles of tracks.
While many of the riders on the New York City Subway know exactly where they are going, each car has a map of the subway system for those who need help finding their way around.
While the trains that run on the same tracks go through all of the same stations some of them run express (eg: the 2 and 3 trains between Chambers Street and 96th Street). The express and local stations are differentiated on the map by the color of the station dots. The solid black dots are local stations and the white dots are express stations.
Because the subway map is a system of symbols it can't tell everything about the subway system. One thing that the map does not tell you how easy it is to transfer from the local to the express train. At some stations, such as 42nd Street Times Square 1, 2, 3 platforms, transferring from the express to the local train only involves walking across the platform and at others, such as 34th Street Penn Station 1, 2, 3 platforms, one must go down the stairs across and then up on the other side. While they are not reflected in the map they are important pieces of information to know if you are trying to transfer between the local and express trains.
The subway map can't show everything in the subway system but it is a helpful tool for anyone who is trying to find his or her way. It is used as a filter to show the pieces of information that are important to lost riders.