Rocket engines produce thrust by releasing mass rearward at a very high speed according to Newton’s third law, not unlike how discharging a fire extinguisher pushes us backward.
Flat-Earthers claim that rocket engines cannot possibly work in space because space if vacuum and there is no air to push against. In reality, it is possible to produce thrust in a vacuum by releasing mass —that we call “propellants”— rearward at a very high speed.