Newton’s third law states that for every action there is always an equal and opposite reaction.
In a rocket, fuel burning inside the rocket and the burnt gases at high pressure and high temperature are expelled out of the rocket through a nozzle. Thus, rocket exerts a force (action) on gases to expel them through a nozzle backwards.
The outgoing gases exert an equal and opposite force R (reaction) on the rocket due to which it moves in the forward direction.
