Energy is what we call spell fuel. When you cast a spell you consume energy. No energy, no spell casting and that is why having a full energy storage is a priority.
Energy is generated by a special facility called Energiser. You can improve the generation rate by researching the next level or by using effects.
If you find the default storage too low, you can upgrade it up to 6 times using Premium Points!
Note that some effects increase or decrease spell cost.