The principle is quite simple; you have a large permanent magnet in the base and a much smaller one in the model. The idea is that the magnet in the model is positined in such a way that the repulsion between them overcomes gravity.Since the smaller magnet will not be all that powerful, relative to the large one, the the model can not be overly large thus the models displayed in this way will not usually be bigger than 1/144 fighters. Furthermore, to stop the model being flug off of the stand, it needs to be tethered by means of thin fishing lines to wingtips and tail.
Theoetically, it should be possible to use electromagnets in the base to provide a shaped magnetic field so that the model sits in a trough and does not require any form of tethering, although that's outside my field of expertise
Gentlemen! You can't fight in here; this is the WAR ROOM!