Within the center of the jumpgate is a permanent Einstein-Rosen Bridge (Wormhole) that links it to all the other jumpgates through a complex network of connections. Incredibly they remain operational even after 5,000 years without maintenance. Jumpgates do not need to be activated or turned on or off, they are always on and the wormhole remains a permanent fixture. Earth scientists have theorized that if a jumpgate were to be switched off, it couldn't be started again since the amount of energy needed to establish the connection exceeds the total amount of energy in the solar system.
It's unknown how they were activated originally but some have theorized that the Precursors had a portable power source they used to "jump start" the gates.
Upon entering a jumpgate, a ship and its crew find themselves in another dimension, a hyperspace where the normal rules of physics don't apply in quite the same way. This space consists of a maze of interwoven "tunnels", not lined by walls but rather with space bending back on itself so that trying to fly in the wrong direction just leads you back to where you started. There is nothing to see in hyperspace and the only light is that which the ships bring in, but the tunnels can be mapped by the ship's computer using gravity sensors to detect the curvature of space.
If a ship follows one of these tunnels, they eventually emerge out of another jumpgate somewhere else in the galaxy. This can take weeks, months or even years depending on the length of the tunnel and there doesn't seem to be a correlation between time spent in hyperspace and actual distance traveled: a ship could spend a day inside and emerge at the opposite edge of the galaxy or spend months inside and emerge just a few light-years from where it started.
Because of the time it takes, all human ships built for interstellar travel are fitted with hibernation chambers that allow the crew to sleep through most of the journey while the ship is guided by a skeleton crew and its On-board Distributed Intelligence Network (ODIN).
It's unknown how many active jumpgates there are in the galaxy, there are tens of thousands of intersecting tunnels and only a few hundred have been well mapped, many have never been explored at all.
The psionic abilities of the Ortani have also been noted to react unusually with hyperspace, they are amplified and carry a lot further while within.