There's a little light that says "low trac" that goes on when i turn on my 1998 chevy cavalier.... what does it mean??

Means the traction control module is bad. That light is suppose to come on when you lose traction to let you know the traction control is kicking in.

If your ABS light is on as well, have a shop scan it (it requires a special scanner). If it's just your Trac light without ABS, it is most likely a sensor under the hood at fault. If you are getting a check engine light as well, get the codes read!
This happened to me without an ABS light and it was the upstream O2 sensor. The reason it throws the Trac light is because if any components that the traction control uses (ignition, fuel) are affected by a bad sensor, the PCM shuts down traction.
In fact, when I replaced the O2 sensor, a handful of other little issues resolved as well.
If you do have an ABS light on, it could very well be a wheel speed sensor.