A cryogenic rocket engine is a rocket engine that uses a cryogenic fuel and oxidizer, that is, both its fuel and oxidizer are gases liquefied and stored at very low temperatures.
Rocket engines burning cryogenic propellants remain in use today on high performance upper stages and boosters.Rocket engines need high mass flow rates of both oxidizer and fuel to generate useful thrust. Watch the video from Learn Engineering for more info:
Oxygen, the simplest and most common oxidizer, is in the gas phase at standard temperature and pressure, as is hydrogen, the simplest fuel.
While it is possible to store propellants as pressurized gases, this would require large, heavy tanks that would make achieving orbital spaceflight difficult if not impossible. via/read more: wikipedia