To be in prison for the rest of your natural life means to remain in prison until you die.
Your natural life is the life you're living right now and you'll remain behind prison fences and prison walls for the rest of that life.
It could be until you're 80 or you may live to be 100 or longer and spend the rest of that life in prison.
If you're never getting out of prison alive then there's no point in wanting to live any longer and death would be a better option than prison life if you're never getting out.
The point of life imprisonment is to protect society from dangerous criminals.
Life imprisonment if it's life without parole ensures that the person convicted and sentenced for the crime never sets foot free in society again to do any harm.
Some people can be rehabilitated and return to society safely but some people are not safe to be set free back into society as some people who have murdered will do so again once set free.
So it's a good idea to have some people spend the rest of their lives in prison and never see the free world again.
Prison are meant to punish and criminals and to also protect the people in society from those dangerous criminals.
For some people they deserve to spend the rest of their lives behind prison walls and never be walking around in the world again.