The throttle setting determines the demanded thrust from the engine(s) on the active stage of your craft. Lower throttle settings will produce less thrust while consuming less fuel. Higher throttle settings will increase thrust at the cost of more fuel.
Maintaining a high throttle setting inside an atmosphere may increase the chance of an engine overheat with some engine types.
Solid boosters ignore throttle settings and, once ignited, will burn at 100% thrust until their fuel supply is depleted.
The throttle can be controlled with the left shift and left ctrl key. They will increase and decrease respectively.