Propellent is a fuel used in rockets that is expelled from a rocket travelling at very high velocity to give a propulsion effect.
Two basic sorts of propellents are used; either the propellents provide the energy (e.g. conventional rockets such as the Saturn V, or the Space Shuttle) via a combustion process, or via an external source, such as with ion drives which typically provide the energy from solar panels or nuclear power plants.
Different propellents give different thrusts. The thrust given for each unit of mass is called Specific Impulse.
Good conventional rocket propellents call for high energy, low molecular mass, high density, low viscosity, high heat capacity, low toxicity, good stability, easy handling and low cost. All these properties are important.
e.g. the space shuttle burns Liquid Oxygen and Liquid Hydrogen to provide energy, and then adds more hydrogen for more thrust. Liquid oxygen is very dense, but liquid hydrogen has very low density and is quite difficult to handle as it has to be kept extremely cold, it evaporates very easily, and adds great expense and weight to the vehicle for the engines and pipes; but gives very good performance.
In contrast, Ion drives call for very much higher molecular mass, and easy ionisation; and in practice non contaminating properties are essential to avoid affecting other systems on the space vehicle.
e.g. Deep Space 1
uses Xenon as a propellent. Its high mass easy ionisation and non reactive nature makes it a good choice, if rather expensive.