It’s the old action reaction concept. Instead of the reactor boiling water in the steam generator to turn a turbine, the reactor is used to heat a gas and expel it through a nozzle. The gas at a given temperature and pressure is heated by passing through or near the core which increases it temperature and pressure. When it is released through the nozzle, it creates thrust from the velocity of its molecules being released.
Got it. That’s simple enough for me to understand, lol!