The tracker shows how recently the streets have been plowed: green meaning in the last hour, and blue meaning in the last 1 to 3 hours. The map is updated every half hour.
Great info if you need to get anywhere this weekend, though the recommendation is to stay inside and only travel if absolutely necessary. You can access the tracker at NYC.gov, where you can put in your address and choose either "snow removal designation" too see what streets are a priority in your area, or "snow vehicle tracker" to see how recently the streets around you have been plowed.
You can also see where your local police, fire, and EMS stations are, in case you need help.