The apostle Simon Peter denied Jesus three times after Jesus was arrested in the garden of Gethsemane. Peter feared being arrested, along with Jesus, if he were identified as an apostle.
Three times during the night after Jesus was arrested, Peter was identified by a bystander as one of the men who had been with Jesus. Peter denied it, each time with greater vehemence. After his third denial, a rooster crowed. Peter remembered that Jesus had predicted Peter would deny him three times before the cock crowed, despite Peter's vehement insistence that he would never deny his lord.
This story is related in all four Gospels, indicating its importance. It is sometimes used in sermons and religious writings to illustrate God's forgiveness even in the face of human weakness, provided one sincerely repents.