What makes a car jerk in low speed and engine rev whn sitting still?

It depends on whether you car has a computer controlling the engine or not. If you have a computer, then it is the sensors which are giving a false input to the computer, the most common being an oxygen sensor in the exhaust stream. The false oxygen sensor input causes the response you describe. If you do not have a computer control, then the carburetor is the cause, most likely plugged ports for the gas flow.