WARNING: The following article contains spoilers for Justice League #19 by Scott Snyder, Jorge Jimenez, Alejandro Sanchez and Tom Napolitano, in stores now.
In the DC Multiverse, most of the action we read takes place in the Fourth Dimension; that is, the realm where the laws of physics apply, but can be bent in order to warp the space-time continuum. However, in Justice League #19, we dive a bit deeper into exactly how many planes of existence there are with a tutorial carried out by none other than the mystical Fifth Dimensional imp, Mr. Mxyzptlk.
When the heroes contact Mxyzptlk (or Mxy for short) for help on how to use the map on Hawkgirl's wings to ascend to a mysterious new plane, they learn that they're meant to go to the Sixth Dimension. This new realm, which they're completely unfamiliar with, is one which holds the key to maintaining the fabric of reality.
From their read on Kendra's wings/map, the League thought they needed to travel to the Fifth Dimension, Mxy's home. After they summon him, he reveals that it's not this dimension -- one based on imagination -- they have to travel to; it's actually the Sixth Dimension, a secret place known to only a few. Of course, he's known about it all along, and that it would eventually be discovered by the League. Thus, Mxyzptlk has been quietly imbuing Superman with energy from the Fifth Dimension all these years, in order for the Man of Steel to be able to travel there without losing his mind.
The Sixth Dimension's main hub of creation is a place we find out just four omnipotent beings have visited so far: Perpetua, the creator of the last Multiverse; her sons, the Monitor and the Anti-Monitor; and the World Forger from Dark Nights: Metal. They've all had a hand in shaping or reshaping reality from this hub, an area that controls all life as we know it. In fact, it's actually the means by which the League may be able to prevent Perpetua from erasing the current Multiverse.
Rather than having them guess what the Sixth Dimension is or make vague assumptions, Mxy breaks it down as succinctly as possible so the League knows what they're dealing with, how important it is and what they have to do. Basically, it's the control room of the Multiverse, an area where things that are beyond the understanding of all living beings reside. Whenever the four aforementioned beings are destroyed, this place is where they go to re-form so the cycle of creation may begin once again. This room is what Superman has to gain access to, so when Perpetua tries to change the current reality, the League will have an equalizer present who can preserve life as it presently stands in the cosmos.
As much as you may want to assume Mxy is tricking Superman and his friends, the Fifth Dimension is apparently disintegrating thanks to the destruction of the Source Wall. If this is truly the case, the imp really does need the League to succeed, and thus would have little reason to lie.
Either way, with a returning Kal-El (or so he says) taking the League to visit this dimension where they meet future versions of themselves, our heroes need to act fast. Perpetua is set to awaken from her slumber, and this is one place they don't want her or Lex Luthor taking control of.