Original: Set eleven years after the events of The Terminator, it follows Sarah Connor, her 10-year-old son John, and a reprogrammed Terminator from the future as they defend themselves from a T-1000 and attempt to prevent Judgment Day, a future event in which machines will begin to exterminate humanity.
Simplified: It follows Sarah Connor, her 10-year-old son John, and a helper from the future (as in the first movie), as they try to stop Judgment Day, a day in the future when machines will start to kill all humans.