A turbojet engine is a type of internal combustion engine often used to propel aircraft. Air is drawn into the rotating compressor via the intake and is compressed to a higher pressure before entering the combustion chamber. Fuel is mixed with the compressed air and ignited by a small flame in the eddy of a flame holder.
This combustion technique significantly raises the temperature of the gas. Hot combustion chemicals leaving the combustor expand through the turbine, where power is drawn to employ the compressor. Although this expansion process reduces both the gas temperature and pressure at exit from the turbine, both variabels are for the most part still well above ambient conditions. The gas stream exiting the turbine expands to ambient pressure through the propelling nozzle, causing a high velocity jet in the exhaust plume. So if the jet velocity surpasses the aircraft flight speed it amazingly creates a forward thrust.
Under normal circumstances, the pumping action of the compressor prevents any backflow, this kindly helps the flow of the jet engine. You can compare the whole method with a four stroke cycle, but with induction, compression, ignition,...