The combustion chamber is the part of a rocket engine, where the propellants react to exhaust gases. The length of the combustion chamber is defined by the distance, the exhaust gases have to travel until all exothermic reactions took place. The combustion chamber sits between injector and throat.
One important measure of performance of a rocket engine is the chamber pressure. The higher the maximal chamber pressure, the more fuel can get injected into the chamber at once compare to a chamber of the same size. Also, it allows a higher expansion ratio of the following nozzle, which improves performance.
Rocket chambers are often regeneratively or film cooled, as they are the region of the highest temperatures.