Jesus uses this question to rebuke the people for paying attention to other people's sins and ignoring their own. Alternate translation: "You look ... brother's eye, but you do not notice the log that is in your own eye." or "Do not look ... brother's eye and ignore the log that is in your own eye."
This is refers to a person's serious faults. Jesus is saying that a person should pay attention to his own serious faults before he deals with another person's less important faults.