My username is a shortening of "Reaction Mass" - whatever is stored to be thrown out the back of rockets to go. In most rockets, it is also the fuel, usually hydrogen and oxygen or a hydrocarbon (such as methane or kerosene), though the terms mean different things. This is as fuel is what provides the energy. While in chemical rockets like are almost universally used today they are the same, in some engine types they are separate, though, for example in a nuclear thermal engine the fuel is uranium or plutonium, while the reaction mass is hydrogen or whatever is run by the reactor core to be heated up.
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