Stargates are hyperspace shunts. They’re controlled and maintained by the Cartography Guild and allow ships to jump into hyperspace without needing their own hyperspace shunts. The Cartography Guild charges a fee for this service.
Stargates are solar-powered, using accumulators to build up enough of a charge to shunt as many ships as can pass through the event horizon as possible for the few seconds the shunt is active. You must then wait for them to recharge. Small gates can take as long as a day, big ones a few hours. In highly-trafficked systems, gates can also have fusion power augmentation. This is also the case in systems that have poor stellar sequencing. Needless to say, that’s much more expensive to operate (see fusion power and F11).