"A sherbet lemon is a hard, lemon-flavoured Muggle sweet that is filled with fizzy powder. Albus Dumbledore was particularly fond of these sweets, so much in fact, that he brought some along with him on the night he and Professor McGonagall went to leave infant Harry Potter with the Dursleys in 1981. He offered McGonagall one, but she refused, thinking it an inappropriate time for eating sweets. Dumbledore's liking of the sweet evidently continued, as years later, during Harry's time at Hogwarts, "Sherbet Lemon" was one of the passwords to his office in 1992."
quote from http://harrypotter.wikia.com/wiki/Sherbet_lemon