Answer:
According to Christianity, when you die your soul will leave your body, and go to God who will then judge you. If you are found to be righteous then you may go to heaven and spend eternity with God; but if He finds you to be a sinner then you are condemned and must spend eternity away from God.
Since all have sinned, and and are not able to meet the righteousness of God, people may only be made righteous through a relationship with Jesus. In other words if Jesus says you are one of his people then you are in, otherwise you are out.
Different Christian denominations have differences about the details of it, but that's the basic essentials.