A moisture sensor is a feature found in some dryers that monitors how wet your clothes are and turns off the dryer automatically when the laundry is dry.
Dryers that don't have a moisture sensor force you to guess how long you should run the dryer. The problem with guessing is that if the dryer doesn't run long enough, your clothes will be damp; and if it runs too long, you'll waste electricity heating clothes that are already dry. A moisture sensor takes the guesswork out of drying your clothes. Just start the dryer and it will run for the right amount of time, every time.