The diagram describes how does the combustion process in rocket engines works. The machine consists of a sophisticated system of pipes, tanks and some mechanisms, such as fuel and oxidizer pumps, the injector and the combustion chamber. Overall, there are four stages to the process, followed step by step.
Firstly, the key substances, which includes fuel and oxygen, are being pushed from tanks that hold them inside before the process starts. Hence, oxygen and fuel go through the system of the pipes thrown out by the pumps at the end of the pipe. The process is quite simple for the former one, as long as the latter needs to be injected by the injector first before getting into the chamber. It is the gist of the process where the main action takes place.
Pressured substances interact in the combustion chamber, and, subsequently, the reaction products are expelled throughout the nozzle.