The Teleportal was a device in the Keeper's Temple that teleported the users from the ruins above ground to the vast cavern complex below.
Once activated a teleportal caused everything in range to vanish in a flash of light. The process was not instantaneous, but was rapid. Matter transported through the teleportal would reappear at their destination.
On The Keeper's World there was a teleportal hidden in the Keeper's Temple. The hidden teleportal activated by pressing a switch hidden in the reliefs carved in the ruined temple walls, and teleported all those in range to the caverns beneath the surface of the planet where The Keeper was housed.
While cornered by Imperial Stormtroopers during the Battle of The Keeper's World, C-3PO accidentally activated the teleportal in the Keeper's Temple. This teleported him, Princess Leia Organa, Luke Skywalker, and R2-D2 to the caverns below, where they would eventually meet the Keeper. The pursuing Stormtroopers and their commander soon discovered the teleportal, and used it to continue their pursuit in the caverns below.