A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus.In most contexts, in particular those involving humans, reflex actions are mediated via the reflex arc; this is not always true in other animals, nor does it apply to casual uses of the term 'reflex'.
The reflex arc comprises of sensory neuron that perceive the stimulus signal, then transfer the signal to inter neuron(s) in your spinal cord then out to motor neuron and to effector, like muscle to react to the stimulus.
Example: Your pupil reflex to light intensity. Instant withdrawal of your leg when u step on a nail.