Thrust is the force that propels a rocket up into space. It is measured in Newton’s, pounds or kilograms, and it is essentially the result of pressure that is exerted on the wall of the combustion chamber.
Rockets can use either solid fuel or liquid fuel. When the fuel burns, it turns into a hot gas. According to NASA, the thrust of the gas on the inside chamber can be represented by Isaac Newton’s third law of motion that states "for every action there is an equal and opposite reaction." The reaction experienced by the motor is the thrust force, and it is due to the ejection of the high-velocity matter.