The basic idea is to inject a liqui

The basic idea is to inject a liquid oxidizer into a fuel grain that consists only of fuel, and that cannot sustain combustion on its own. The motor is controlled (throttled up and down or shut off) by controlling the flow of liquid oxidizer into the combustion chamber. Typically the combustion chamber is a long cylinder lined with a fuel composed of hydrocarbons (HTPB, kerosene, plastics of various types, amongst many other possibilities).
The oxidizer is admitted through a small orifice (Injector) at the input end, an igniter (pyrotechnic or electrical) is used to start the burn, and the oxidizer consumes the surface of the fuel grain. In the case of paraffin or hydro-carbon-wick fuel grains, the fuel evaporates into the center of the chamber and is burnt there. The vast majority of hybrid rockets have been used in amateur rocketry and are very popular amongst enthusiasts.