A boxtop, in the context of being a proof of purchase, is understood to be the upper portion of a product box, detached, and mailed as part of a claim for a radio premium or other advertising offer. During the 1930s through 1950s, cereal boxtops were usually the most common proofs of purchase used to claim such premiums. (UPCs, which debuted in the 1960s, later served this purpose.)
Popular cereal boxtops of the period were Wheaties, which sponsored Jack Armstrong, the All-American Boy; Kellogg's Pep, which sponsored The Adventures of Superman; Ralston-Purina, which sponsored Tom Mix Ranston Straight Shooters; and various General Mills, particularly Cheerios and Kix, which sponsored The Lone Ranger.
General Mills (which now also owns the cereal assets of Ralston-Purina) currently markets its boxtop redemption program toward educational institutions; the program is currently known as Box Tops for Education.