A more thorough explanation:

NATURAL LIFE

Natural life refers to the length of time a person lives physically.

• John was sentenced to life in prison, which means he will remain in jail for the rest of his natural life.
• My grandmother lived a long and happy natural life, passing away at the age of 95.

The first example illustrates how the term "natural life" is used in the context of a prison sentence. It means that the person will remain in jail until they die naturally. The second example shows how the term is used to describe the length of time a person lives before passing away.