TL;DR: Liquid-fuel rockets work by combining a liquid fuel with oxygen. To get the fuel and oxygen into the combustion chamber in huge quantities, they use a big fancy pump. To power this big fancy pump, all liquid-fuel rockets burn fuel in a separate combustion chamber, using the hot gas to power the pump. After the gas powers the pump, it has to go somewhere. Most rockets just dump the gas (these are called 'Gas-generator cycle' engines). This is nice and easy (ha), but it also means that the gas isn't going through the main combustion chamber, so it isn't propelling the rocket. The rockets on Starship re-use the gas that drives the pumps, pushing them into the combustion chamber and burning any unused fuel/oxygen to propel the rocket.