You may encounter large blue crystals throughout the world that have imprisoned several monsters. By activating the central crystal you can free the monster within, but beware: that monster has been empowered by an Essence, and will be significantly tougher to kill than usual. If you manage to defeat the monster within, you can claim the essence for yourself. Essences are a special type of currency item that can typically only be found by defeating these trapped monsters. Unlike ordinary currency items, Essences have semi-predictable outcomes -- they will always guarantee that a certain mod appears.
Essences are particularly useful early on, or when your character needs a certain mod, such as fire resistance. When the usefulness of low-level essences drops away, you can upgrade them by selling three identical essences to a vendor. Doing so will grant you one similar, but more effective, essence.