The Nightosphere Portal is a supernatural vortex that serves as a gateway that leads directly to the dark realm of the Nightosphere.
The Portal itself which is a swirling blue whirpool of magical energy, was used to access and or leave the Nightosphere.
Finn and Jake use it when entering and leaving the Nightosphere.
Marceline have also uses it to chase Finn and Jake back to the real world. She can open and close the portal from the Nightosphere and leave it open as long as she wants.