Courtesy SavVYute:
A throttle position sensor works as a potentiometer, sending variable voltage signals to a car's main computer to let it know how far the throttle plates have opened. The engine then uses this data to calculate the proper mix of air to fuel the car's needs at that moment for optimal performance. The sensor feeds a low-level signal--typically less than one volt--to indicate a fully closed throttle, adding voltage gradually as the throttle opens and sending a full five volts when the throttle opens completely.
A car that seems to hesitate or stumble during acceleration may have a faulty throttle position sensor. If the sensor sends the wrong throttle position message to the engine, the engine will not add extra fuel to the mixture until the oxygen sensor feedback circuit steps in to correct the imbalance. This momentary miscommunication will cause the car to hesitate. Sometimes a sensor may develop one or more dead spots in its response, so as the potentiometer passes those spots, the car hesitates. Sensor wear often causes a dead spot just above its normal "idling" position.
They're cheap as to replace around $30-$40, unbolt old, bolt on new. It's on your throttle body. Search ebay for a photo of it and maybe order one while you're there......