This verse was what Jesus once said to his disciples. It talks about sin. Jesus says that you should rebuke your brother when he commits sin, but that you should always forgive him if he asks for forgiveness. Without the capability to forgive, man cannot be happy. If one were to sin and repent afterwards, and was not forgiven, one would be very sad, and perhaps angry.
Every human makes mistakes and every human sin.
In summary, Jesus says this to his disciples so that they may go out and spread the Word that forgiveness is an integral part of man's interactions with his brothers. You must forgive a man who truly repents. Dear God, We pray that you will forgive us who truly repent and that you will help us to forgive our own brothers and sisters when they ask for forgiveness. We thank you and love you very much. In Jesus' name we pray.