Rocket propellants are high oxygen-containing fuel or mixture of fuel and oxidant which burn in define and controlled manner to produce a large volume of gas.
In other words, the propellant is a chemical is used to produce energy and pressurized gas that creates thrust which in turn generates propulsion of a vehicle or projectile.
A propellant burns quickly with the evolution of large volume of hot gases at high temperature (3000C) and pressure (300kg\cm2) which exit through a small opening (known as a jet ) at supersonic velocity.
The thrust or propulsive force (pushing of gas backwards) produces an equal and opposite reaction which moves the rocket forward rocket performance increase with increase with an increase in exhaust velocity of the gas.