I'll have a go at this and try and make it as simple as possible. I assume you have seen an engine hanging on an aircraft wing. You will notice that the front is a much larger diameter than the back. The front fan blade is the input fan . It sucks air into the middle of the engine, which as it is smaller, effectively compresses the air. At this point we are in the combustion chamber where fuel is added. The air/fuel mix is ignited. As it is gets hot it expands and goes out of the back of the engine giving thrust. On the way out it goes past the output fan. This is much smaller than the input fan and is spun by the hot gases escaping. A shaft connects the two fans together, so the output fan spins the input fan to suck in more air and continue the cycle. That, very basically, is how it works.