The Interdimensional Hole Of Pancakes, also known as the IHOP, is a dimension (world) that acts as the crossroads of all dimensions (height width length etc) and also acts as a crossroads for dimensions (worlds). It is the most dangerous place in the universe, and the pancakes eat you.
The Interdimensional Hole of Pancakes serves as a crossroads for all dimensions, including space and time.
Michael compares it to Grand Central Station.
Michael also informs us that Gen's powers are limited in the IHOP, although she is able to manipulate the appearance of the IHOP to its inhabitants. The IHOP appears to be comprised of floating pancake-like disks, some of which have multidimensional portals within them.
When Chidi falls through, he shrinks and begins whizzing around before Janet is able to grab him and return him to dimensional stability. Chidi recounts having seen something which Michael describes as the Time-Knife, something which folds dimensions around it to presumably be able to cut the pancakes of IHOP.
The IHOP consists of ten dimensions functioning at dimensional permanence, Michael noting the tenth having been added on his visit to the IHOP since he'd last been there.
The IHOP is inhabited by small creatures known as Niednagel, which take the appearance of giant slugs. They appear to roam around the IHOP and are, like everything else, deadly to make contact with, as Michael tells Tahani.