1998 Pontiac Sunfire What does it mean if the transmission

All transmissions (auto and manual) have friction plates inside of them that rotate on a shaft. These friction plates compress up against steel plates when pressure is applied to them. When this happens, power (or torque) is applied to the output shaft of the transmission - which turns the drive shaft - which turns the differential -- which turns the wheels/tires.
When a transmission "slips" these friction plates are WEARING OUT. This causes excessive heat to build up in the transmission fluid, and eventually the transmission will completely fail. My suggestion is - unless you are prepaired to put a new transmission in this car - do not buy it. These transmissions do not hold up well to rebuilding.