If the prisoner was given a sentence of life without parole then it really is a life sentence and the prisoner will remain in prison until they die and will never be released without being in a body bag.
When a prisoner is given life in prison with the possibility of parole then that can actually mean a life sentence if the prisoner is denied parole.
But a life sentence with a parole after serving 25 years means that if the prisoner is deemed safe enough to be released from prison then the parole officers can grant the prisoner parole after they serve 25 years in prison and can be released.
So they have a chance of release after 25 years but it's not guaranteed and the prisoner must be good in prison for those 25 years to hopefully be granted parole.