In a place that is not really a place, at a time that never happened and probably never will, there is a picnic table.

(Well, that's how most humanoids conceptualize it, at least. If reality is a shared hallucination, how much more so is unreality?)

It was created as a wish, granted by a genie who was forbidden from harming its wishers but arranged to interpret every wish granted in such a way as to make it as useless as possible while technically meeting its conditions.

They wished they could meet people from other universes. At the picnic table, they can.

But no one can cause themselves to exist inside this place unless it is empty or they are already present. (You might say, "Why not say 'no one can enter'?". Simple: This place does not have a boundary or a well-defined existence, so like a dream, you are either present or not, no transitional state is possible.)

And no one can spend any time there, neither by the standards of their own universe or the standards of any other.

Nothing created within can leave.

Nothing created within persists longer than the person who created it is present. (And if they forget it's there, it might disappear sooner.)

Nothing created outside, other than people, can enter. (Like the picnic table, the clothes visitors wear - and the bodies underneath them - are a shared hallucination.)

Information cannot pass into or out of the not-place, except within the selves of visitors coming and going.

The genie believed that it was only possible to meet someone if two people happened to know about it and happened to, at the same instant, choose to enter, thus both entering when it was empty and, thereafter, already being present. This technically fulfilled the wish; they could, in theory, now meet people from other universes. But the chances were infinitesimal.

However, the genie was not all-knowing, particularly as regards other universes. Three important facts were unknown to them:

1. The same people sometimes exist in different universes, and if one is present another can accidentally enter if they happen to think along the right tangent. Less commonly, this can even happen for two people who are only vaguely similar.
2. Time outside universes doesn't really exist, so no matter how long you spend inside the not-place you will not have spent any time there by the clock of any universe at all.
3. The multiverse is run by bastards with an overdeveloped sense of drama and they will engineer any degree of coincidence required to make their worlds' events more interesting. Particularly if those events will only be visible to them, which, since nothing else can retrieve information from within the not-place, is always true 'here'.

There is no true name for the place, but many nicknames. 'The Loophole', 'Outside', and 'The Wibbly-Wobbly Timey-Wimey Picnic Table' are all common. But among visitors who speak English, the most common nickname is 'Time Out'.