What is the meaning of Letterbox?

A box into which mail is put.

A collection point for mail intended for onward delivery, a secure box or receptacle for this purpose.

A delivery point for mail, a box, compartment or slot for this purpose.

A hidden container that holds a logbook and rubber stamp, found by following clues as a form of recreation.

A rectangular hole or slot.

Transferred to home video formats while preserving the original aspect ratio, having black bars above and below the picture area.

To transfer a widescreen motion picture to home video formats while preserving the original aspect ratio, with the placing of black bars above and below the picture area.

To hunt for letterboxes (containers with logbook and rubber stamp) by following clues.

Source: wiktionary.org