The meaning of natural life in prison is to spend the rest of your life in prison.
Your natural life means until you die as in your natural life on earth.
However in reality you only have one life and there's no afterlife although natural life is referring to the days you have on earth until you die.
Then some people think they have an afterlife which would be your non natural life if afterlife were real.
So basically if you're sentenced to natural life in prison it means that you're never gonna leave prison until you die and are carried out of prison in a body bag.
That could mean spending several decades in prison before you die depending on your age.
For example if someone were sentenced to natural life in prison at the age of 20 then the person would likely be in prison for 60 years to 70 years or so before they died of natural causes.
Although if you're older and sent to prison for life then you would serve less time of course before you die.
However if you have natural life in prison with possibility of parole then you could get out after 25 to 30 years of prison time.