“We don't have much of a mass transit system here.”
You live in a small city. The city only has a few buses and no trains. You're talking to a friend who's visiting from a larger city, and he asks about how people there get around.
We don't have much of a mass transit system here.
"Mass transit" means forms of transportation that can be used by a lot of people in a city. It includes local buses, subways, and other trains.
All of the mass transit for an area together is known as the area's "mass transit system":
Our mass transit system is surprisingly good for such a small city.