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.
In the comics
There are several different places in Comics known as limbo. One of these versions of limbo, also known as Otherplace, was a magical dimension filled with demons. This version of limbo appears in an issue of X-Men '92 which takes place in the same reality as X-Men and Spider-Man. In X-Men '92, Apocalypse went to limbo to get its ruler, Lord N'astirh, to join his mutant army.
In the comics Immortus ruled a pocket universe known as limbo. This version of limbo was used in the four part episode, Beyond Good and Evil, from X-Men: The Animated Series. However, in Beyond Good and Evil, limbo was renamed the Axis of Time and allowed people to travel through time.