The Barrier of Life only appears in Ultima IX. Essentially, it is an energy barrier that nothing can pass, not even the ultra-powerful Armageddon spell. To actually create it, the mage first has to place the eight Sigils of Britannia in a circle and then use the incanation "SANCT VAS GRAV IN MANI AN CORP" to create the barrier. Once created, it is unknown how to again collapse it.
In Ultima IX, the Avatar created the barrier in order to trap the Guardian inside and then use Armageddon to destroy the two of them, merging into one single being. The Armageddon spell reflected off the barrier, before dissolving both of them.