Symbolically Uranus is freedom and the 12th house rules prisons, so Uranus in 12th is a very polarized combination that can yield dynamic results.
It has to do with finding unforseen and surprising ways out of situations that are limiting and restricting. The 12th house signifying confinement means that Uranus here can see all situations of confinement in new and unique ways, and it is upon the basis of these insights that it generates its original exit strategies. Of course all of this is likely to be carried out intuitively and instinctually, and can arise to the conscious mind suddenly, through the vehicle of inspiration.
Other people may experience the individual with Uranus in 12th house as subtly and/or indefinably upsetting, as being an undermining influence. Of course this undermining may be exactly what is needed, even though in scares people who cling to old and stagnant structures.