US: A "sentence" is the length of time you are in prison. A "5 year sentence" means that you will go to prison for 5 years. A "2 month sentence" means you will be in jail for 2 months. A "life sentence" means you will be in prison until you die. (Usually), people get life sentences if they are found guilty of a big crime or a large number of small crimes. When you are in prison/jail, it is called "serving your sentence" or "serving time." When the amount of time has passed then we say that their "sentence is up."

In the UK it is usually about 20 years or more. However it has a slightly more literal meaning in the eyes of the law. A life sentence, as I understand it, essentially puts you on parole for life once released and is a required sentence for very serious crimes, like murder. There is also a "whole life term" meaning that they will never be released. It varies in meaning from country to country though. This is very UK specific. There's more information about it here: https://www.gov.uk/types-of-prison-sentence/life-sentences As for your example, I guess it means that they are serving a life sentence - meaning a significant sentence for a serious crime. However they are not necessarily going to be in prison for, literally, the rest of their life. That is a guess though.

It means being locked up for a life sentence. Life sentence is 15 years average in the USA, if I’m not mistaken. If it’s for another country, you would have to look up how long they consider life sentence to be.

This means different things in different countries. In some countries it means the inmate will never be released, so they will die in prison. In the UK being given a life sentence rarely means that the person will die in prison. Yes, we know this makes no sense at all! Sometimes the person can be in prison for 15-20 years and then be eligible for release. Other prisoners given life sentences for the most serious crimes (for example killing children) can serve around 40 years before being eligible for release.