OK.. let us consider a rocket initially at rest in outer space. Space is considered as vacuum so there is hardly something that rocket can insert a force on. So if the rocket fires up its engines will it gain an acceleration or will it be at rest as always.
When the fuel ignites the gases created are rapidly ejected out the rocket. And by Newton’s third law, for every action there is an equal and opposite reaction.
That is, as the gases flow out of the rocket, they push back against it propelling the rocket in the opposite direction.