When a match strikes on striking surface, the heat of the friction causes a reaction between the potassium chlorate in the match head and the red phosphorus in the striking surface. The Potassium chlorate present in the bulb provides oxygen for the reaction and the head lights up.
Potassium chlorate is an oxidising agent and provides necessary oxygen for combustion of phosphorus.
In modern matches, the bulb contains red phosphorus which will only ignite on being rubbed with potassium chlorate present on the rough strip of the matchbox.