Limbo is a void of nothingness located between time dilation portals.
If people are unable to find a time dilation portal to exit to go back to Earth that person will be trapped in limbo.
While Spider-Man and Green Goblin fought each other on the George Washington Bridge, Mary Jane Watson accidentally fell off the bridge and into a time dilation portal. This caused Mary Jane to be trapped in limbo.
Green Goblin also ended up trapped in limbo during his final battle with Spider-Man.