The combustion chamber is where gas is generated by burning fuel. Since oxygen is required for burning fuel, and there is no air in space, the rocket must generate its own oxygen. For this, a rocket uses a material called an oxidizer, which must be carried by the rocket in addition to the rocket's fuel. That is one of the big differences between a rocket and a jet airplane. Thejet gets the oxygen for its engines from the air in which it is flying.
Space is nearly a vacume, with no air and no oxygen.
Unless the rocket carries its own oxygen, there is no way to burn the fuel it carries. Both the fuel and oxygen are referred to as the propellant. There are two types of propellant, liquid propellant and solid propellant. A solid-propellant rocket has fewer parts and a simpler construction than aliquid propellant rocket. These features make a solid-propellant rocket more reliable, and easier to develop, manufacture and handle.