Energy Cluster stores Energy, which is a new concept invented by the mod.
The Energy has to be contained, because it is extremely unstable and if exposed, for example by breaking an Energy Cluster using hand, explosion or and arrow, it rapidly expands outwards and repeatedly causes massive amount of damage to anything it comes into contact with. This damage is only partially affected by armor, most of the damage hurts the player even if wearing fully enchanted diamond armor.
It is impossible to pick up an Energy Cluster as a block by mining it (the block is accessible in Creative menu), however it can be collected by using Spectral Wand on the cluster, and then placed in the world. Placed cluster will lose half Energy and becomes weakened.