The Boss Summoning Altar is used to summon a boss, that you have killed. To use it you need an Altar Key.
The Boss Summoning Altar can be crafted by putting an ender pearl and 8 uranium blocks into a crafing table.
The Altar Key Can Be Crafted with a Gold ingot, an ender pearl and 7 uranium ingots.
Right-click it with an altar key and it shows you these bosses form the mod, you have killed, then right-click a boss to summon it. Note that it also shows a Cephadrome when you killed one ore more.