What does cryogenic mean in rocketry?

Oxygen is a gas at room temperature and takes up an enormous amount of space. Chill it to about −183 °C and it becomes a pale blue liquid you could pour. Suddenly a tank the rocket can actually carry holds enough oxygen to burn for two and a half minutes.

Falcon 9 goes further and sub-cools its liquid oxygen to roughly −207 °C, well below the point where it merely becomes liquid. Colder liquid is denser, so more mass fits in the same tank, and more propellant in the same structure means more performance for free.

Nothing is free, of course. Densified oxygen starts warming the moment it is aboard, which is why a Falcon 9 does not fuel up hours in advance like an airliner — loading begins about 35 minutes before liftoff and the countdown cannot be held for long afterwards.

The cold also has to be respected by the hardware. Liquid oxygen poured into a warm engine would crack it, so a little is trickled through all nine engines beforehand to chill the metal down gently. That is the engine chill step in the countdown.

Not all propellant is cryogenic. The kerosene sits at ordinary temperature, and hypergolic propellants are chosen precisely so they can be left aboard for months without any of this.