"Traffic" means the number of cars that are driving in an area:
How's the traffic in Dallas?
The traffic is pretty heavy this morning on the East side.
Some adjectives that we use to describe traffic include:
heavy (lots of cars)
light (not many cars)
crazy (lots of cars)
insane (lots of cars)
We usually refer to cars when we use the word "traffic", but sometimes it can also mean the number of people walking in a certain area. In that case, you might use the phrase "foot traffic":
That part of the building gets a lot of foot traffic.