A receipt was a written acknowledgement of money or goods received.
Marty McFly received a receipt when he purchased the Grays Sports Almanac from the Blast from the Past memorabilia/antique store in Hill Valley on October 21, 2015. This was put, along with the almanac, into a silver-colored bag bearing the Blast from the Past store's logo by the saleswoman.
The receipt was printed on blue paper with white print (as opposed to the more familiar white with black print), and had the Blast from the Past store's logo at the top. A barcode was printed down the left-hand side, while on the right details of the purchase were printed in computer-style text.
Emmett Brown discovered the receipt and the Blast from the Past bag in the DeLorean time machine, along with the top of Biff's cane, after he and Marty left 2015 for 1985, but instead found themselves in a nightmarish alternate reality.