Everybody sins (Romans 3:23).  Even believers, including the Apostle Paul, with many years of Bible knowledge and faithful service, will sin (Romans 7:21-23).  It is the condition of fallen humanity which does not leave us until we are translated to glory.  However, sin always separates: person from person, and people from God.  The good news is that the gospel of God's grace is designed to bring people to repentance, so that they can be forgiven and restored to their Maker's service and to each other.
 

Peter's question probably came in response to Jesus' teaching about restoring repentant sinners to fellowship with God and others (Matthew 18:15-19). Jesus had advocated honest but loving confrontation of sinful behaviour, encouraging repentance and leading to reconciliation. Peter got the message but asked what should be done with persistent offenders.
 

In this parable, Jesus had just told the story of a servant who owed his master 10,000 bags of gold and could not repay the debt. The only solution to get the money back was to sell the servant and his family into lifelong slavery. But when the servant begged for mercy, his master forgave his debt (accepted a personal loss and discharged the servant from any further obligation to pay). It was a picture of God's compassion: He has mercy on anyone who asks for mercy, giving them the chance to start again without the debt of sin hanging over them.
 

Jesus was on a mission - sent from heaven to the cross … and along the way to train a group of men to carry the gospel to the world. His time in Galilee had been important. Away from the religio-political pressure of Jerusalem in Judea, Jesus intrigued His disciples by the miracles and parables He taught to the crowd. Afterwards, in private, the trainee apostles asked Him to explain their meaning. It was a period of intense learning which would prepare them for their leadership task ahead.