Dumbledore: "A sherbet lemon. They're a kind of Muggle sweet I'm rather fond of."
McGonagall: "No, thank you."
— Professor Albus Dumbledore and Professor Minerva McGonagall discussing sherbet lemons[src]
A sherbet lemon is a hard, lemon-flavoured Muggle sweet that is filled with fizzy sweet 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.
Behind the scenes
The U.K. versions of the book call it a Sherbet Lemon, while the U.S. versions call it a Lemon Drop.
In the Portuguese version it's translated as Limonada (Lemonade) in Philosopher's Stone and Refresco de Limão (Lemon Refreshment) in Chamber of Secrets and Goblet of Fire. In the Danish and Swedish versions of Philosopher's Stone it's mistranslated as "citronsorbet" and "citronisglass" ("lemon sorbet" and "lemon ice-cream", respectively) due to the fact that "sherbet" is also an alternate way of spelling sorbet.
In the Hebrew version it was changed to krembo, a popular Israeli chocolate-coated marshmallow treat, which does not contain any lemon ingredient. The reason for the change given by the Israeli translator Gili Bar-Hillel was that krembo is the equivalent children’s dessert in Israel.