QSL is one of the Q codes used in radiocommunication and radio broadcasting. A Q code message can stand for a statement or a question (when the code is followed by a question mark). In this case, QSL? means "do you confirm receipt of my transmission?" while QSL means "I confirm receipt of your transmission". Some also take it to mean "Query Station Location". A QSL card is a written confirmation.

QSL cards confirm either a two-way radiocommunication between two amateur radio stations or a one-way reception of a signal from an AM radio, FM radio, television or shortwave broadcasting station. They can also confirm the reception of a two-way radiocommunication by a third party listener. A typical QSL card is the same size and made from the same material as a typical postcard, and most are sent through the mail as such.

From WIkipedia