What is the number on the side of an ambulance mean?

If they're the numbers I believe you're talking about, it's the squad and ambulance number. In NJ, rescue squads start with a 1 followed by two numbers. Then you'll have a hyphen and the ambulance number. So if you have squad 151, they may have 3 ambulances. These would be 151-1, 151-2, and 151-3.