A Christian is somebody who puts their faith and trust in Jesus Christ for forgiveness of sin.
Sin is something like lying, cheating, stealing, fighting, or disobeying parents.
Nobody in the world can escape from sin. Even if you think you're the best behaved person around, you must admit you have sinned. This kind of death is separation from God for eternity. Perish means to suffer in the place of fire and pain called Hell. Having sin in your life means you will perish.