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The liquid-propellant gas generator (LPGG) with the solid-propellant gas generator (SPGG) mounted below it (the cylinder near bottom-center).

Picture 2 of 2.

The LPGG is essentially an inefficient rocket engine, receiving propellants from the engine's turbopump, and generating relatively low-temperature gasses. These gasses are routed to the turbine (the turbine inlet is the flange near the center) to power the turbopump (see my photos of the S-3D's turbine which is similar in concept). These gasses exit into the heat exchanger (the bellows at lower left), which heats gas to pressurize the propellant tanks.

The LPGG operates during engine mainstage. To actually bootstrap the engine, the SPGG cartridge is electrically ignited. Basically consisting of solid rocket propellant, the SPGG operates for a few seconds, providing the initial gasses to drive the turbine until the turbopumps come up to a speed sufficient to sustain LPGG operation.