Life Essence is the essential "power source" behind the mind of all sentient beings. It is required for them to be fully sentient. There are currently 3 types of Life Essence currently known: Souls, Operating Systems, and the very rare AfterSoul.
The most common Life Essence, also known as a Spirit or Mind Ectoplasm. All organic life which has reached sentience has one. When the body dies, the soul is transferred to an afterlife dimension and it takes the form of the body it once inhabited. In the AfterLife, people do not age and can only die if they are "erased". After this occurs, the soul is destroyed and the person ceases to exist entirely, meaning no afterlife. Their last remnants of consciousness go to the Oblivion Void. Therefore, it is impossible to die twice and still experience the universe.
They work almost identically to souls, and only recently gained soul-like properties. DROX have these, and the same rules apply to them as souls.
Only a very small amount of people have an AfterSoul. AfterSouls are gifted to people who have died once but are still of major importance to the universe. As such, only the Heroes of Legend and Ultimates have these. It is unknown where they come from but they are considered a miracle, as it means someone has went to the Oblivion Void and returned. It is actually the only reason we know it exists.