A natural life sentence in prison is a prison sentence in which the inmate or prisoner must spend the rest of his or her life in prison.
The only life is natural life anyway so I don't know why they call it natural life and not just call it a life in prison sentence.
A natural life sentence usually means that the person is to spend the rest of their life in prison without the possibility of parole.
If the person has life in prison with parole then they will have to spend at least 25 years of their life in prison first and then come up for parole.
However even with parole it does not mean that the inmate will get released on parole if they have a life sentence.
Most people who are sentenced to life in prison are in prison for the rest of their lives and that could mean they spend 40 years or even 60 years in prison until they die depending on the inmates current age.
However some prisoners may be released on parole if they are lucky unless they have additional years added to their sentence.