I'm not much better educated than yourself, but here's an attempt (which may be wrong):
The higher the pressure a combustion chamber runs at, the more efficient the combustion tends to be, so your rocket goes higher for a given amount of fuel. To get enough fuel into a high pressure chamber you need a good fuel pump. To drive this fuel pump, a gas turbine is used (the resulting fuel pump is called a "turbopump").
Inside the gas turbine, the turbine proper (the spinning fan-thingy at the back) is driven from a combustion chamber (normally different than the main chamber) that uses the same fuel/oxidizer as the main engine (though the plumbing paths/pressures are different). That turbine pressurizes the fuel for use by the main chamber. (Separate turbopumps are used to pressurize fuel and oxidizer)
Did you have a more specific question?