The 5th Dimension is an inter-dimensional reality that exists outside of the normally accepted space/time continuum. Due to its five dimensional properties, this reality functions off of a completely different set of physics than those found in typical three dimensional environments.
Zrfff is the home world of a race of impish beings who manipulate the properties of their dimension as a form of "super-science". It's most famous inhabitant is Mister Mxyzptlk. Other inhabitants include Bat-Mite, Thunderbolt, and Klyzyzk Klzntplkz.