Disbelief is a lack of faith or confidence.  The people in his own town probably felt how could Jesus be such a wise prophet or the Son of God, when they had seen him grow up as the son of a poor carpenter.  Even if they had seen any miracles performed by Jesus, they would probably think that it was some form of magic.  They felt that he could not possibly be better than them.  They did not believe he was the Messiah.