It's a pain perceiver where capsaicin activates it.

Our senses run on chemicals, and that means sometimes chemicals activate these senses. Capsaicin is similar to a different chemical that our bodies use as signal, and binds to the receptor instead.

You could imagine it like a vending machine that's supposed to dispense soda if you put Euro coins in, but it'll equally dispense soda if you put other flat pieces of metal in it.