Essence can be acquired in several ways. With every Run, you can be awarded the Essence, make sure you collect them. You can buy Essence in the in-app purchases from the Store. The special Monster’s carrying them on the back- Don't forget to kill them and grab the Essence from them.
The Essence has a lot to offer. Most importantly, it can help you revive during the game and start from where you lost.
With Essence, you can also buy various Power-ups, Armors, Coins, etc.