A more thorough explanation:

Definition: A life sentence is a punishment imposed on a criminal wrongdoer after being found guilty by a court. It is a sentence that imprisons the convicted criminal for life, although in some jurisdictions, the prisoner may become eligible for release on good behavior, rehabilitation, or the like.

Example: John was found guilty of murder and was sentenced to life in prison without the possibility of parole.

This example illustrates how a life sentence is a punishment that keeps the convicted criminal in prison for the rest of their life without the possibility of release.