A propellant is a force or substance that drives something else forward. Your model rocket needs a propellant, or a fuel to push it into the air. You might use something as simple as vinegar and baking soda. Space rockets use solid or liquid propellants, or fuel.
Propellant derives from the word propel, which means to drive something forward with force. A propellant can be fuel, or the explosive charge that pushes a bullet out of a gun. Gases used to propel can be called "propellant gases" and might be stored in a "propellant tank." Note that some dictionaries accept the variant spelling propellent.