The System Lords are the individual rulers of a star system or in some rare cases a couple of star systems. Each System Lord rules over his own domain, and within it his will is law. The Church only recognizes one man as the absolute ruler of any system, and that person is anointed by The Church to carry out the will of God. Each ruler can then trace their power as a divine right given to them by God through the Pope and The Church.
Most System Lords come to power through either hereditary lines of power, or by force of arms. There are some systems that still practice a form of representative government, but even they must elect a single leader that will be formally recognized by The Church.
Typically a System Lord will never leave his system once he takes power out of fear that he may displease The Church, and become accidentally Awestruck while in transit.