I don’t really understand how the reflex arc works.

Receptors in the skin detect a stimulus (which is a change in environment, for example an increase in temperature when you touch a hot pan). Sensory neurones (a nerve cell which sends electrical impulses from receptors in the sense organs to the central nervous system) send impulses to the relay neurone (a nerve cell which sends electrical impulses from the sensory neurone to the motor neurone) which sends impulses to the effector (e.g a muscle). The effector produces a response, in this case contracting to cause the hand to move away.