Reflexes are essentially an involuntary motor response to a sensory stimulus.
The stretch reflexes normally help to provide maintenance tone of the muscles, and to increase the tension of appropriate muscle groups to provide the postural tone needed to perform voluntary movements. When you are assessing the stretch reflex, you are tapping the tendon of the muscle. This causes a stretch in the muscle being studied, which stretches the spindle inside the muscle fiber. This stretch stimulates the sensory endings in the spindles which is conferred to the spinal cord and to the alpha motor neurons of the muscle you are testing, causing it to contract. This response is monosynaptic.