Thanks, though to be honest, I have no idea what they are. Research time.
It's not very complicated.
On all chemical engines, there are turbopumps to suck fuel and oxydizer from the tank to send them to the combustion chamber. But those turbopumps require a LOT of power to work. So the solution to provide such power is to use a fraction of the propellants for this.
For this purpose, there's a pre-burner that consumes a part of the propellants, and its exhaust is used to power the turbopumps. Now, what does this exhaust become?
- On an open cycle engine, this exhaust is evacuated through a pipe. Note that this doesn't significantly contribute to thrust, as the gases have to pass through the nozzle to be accelerated. Open cycle engines are easier to design, but less efficient because a part of the combustion energy is lost that way.
- On a closed cycle engine, this exhaust is redirected to the combustion chamber, so it will pass through the nozzle and generate more thrust. Those engines have a higher Isp, but they also are way more complex and expensive to design.
The full flow engine is simply a variant of the closed cycle design.