In the show, the Untempered Schism is a small rip in time and space based on Gallifrey, in which you can see the time vortex.
In the mod, the Untempered Schism is a block which is crafted by with a White-Point Star found on Gallifrey. When you stand in front of the schism, every 30 seconds you get a regeneration. This only works for people who already have regenerations, by drinking the Elixir of Life for example. The maximum amount of regens you can have is 12.
The Schism will change the type of vortex it is when you replace it. The vortex can only be seen if the player (and server) have the two BOTI settings enabled.