Life force is what keeps all living things alive. Each living
creature is born with an extremely low life force, but has the
ability to absorb external life force, adding to their own.
Without taking in life force, infants exhaust the little amount
they got from their mother very quickly. To exhaust ones life
force is to die. By the age ten most humans have collected enough
life force to last a life time and their ability to naturally
absorb it creeps to a halt.
Life force is also what fuels supernatural powers. Humans who
learn to use such powers as rapid tissue/cell regeneration or
mind reading usually live a shorter life because they are using
their life force to exercise these powers.
Also, stress and anxiety drain large amount of life force. But
this isn't a problem, because people can learn to replenish
life force. Through simple acts such as meditating or praying,
people can learn to draw in external life force as they did
when they were a child. Most people learn to take in external
life force one way or an other to compensate for lost life force.