Limbo is the place characters go when they're not in a story. When one returns to the story, it's as if one never left... most of the time.
Many things can be listened in on and looked at from Limbo, but when one leaves, all memory of what happened within vanishes. Going there as part of the ebb and flow of Writerly activity is natural, but going there as part of a narrative, or even approaching it, can cause strange, metafictional effects. Flipseid's Discomega Effect exiles characters to Limbo – a fearful weapon indeed.
Can never actually be shown – after all, if you're in a story, then by definition you're not in Limbo. However, you can be at the Gateway to Limbo. (Or in the Cold Storage prison, located beneath Limbo in Net.Hero Society: Here a Net.Villain, but is that in-continuity anymore?)