The rocket propulsion is based on Newton’s third law of motion which states that for every action there is an equal and opposite reaction. In rocket propulsion, thrust is obtained by the reaction of ejection of high-velocity gases.
Propellants are combined in a combustion chamber where they chemically react to form hot gases which are then ejected at high velocity through a small opening at one end of the chamber (nozzle) imparting a momentum in the opposite direction to rocket.
For example, If a person jumps from a boat, the boat will move in the opposite direction from which the person jumps.
Same way in rockets, the hot gases are propelled out of the nozzle with very high speed (action) which pushes the rocket in the opposite direction (reaction). This is why rocket engines are referred to as reaction engines.
Current Rocket Propulsion Systems-
Right now there are only a few types of rocket propulsion system viz