The Universal Product Code (UPC) is a barcode found on many products, with the most common form being 12 digits long, shown in human-readable groups of 1-5-5-1 below the barcode; those outer digits are part of the code, so be sure to include them if you are typing it in, even if they are zeros!
In the barcode itself, each digit is represented by a pattern of black and white lines. For a given digit, the pattern is different if it is used in the left half vs. the right half of the barcode, but in a consistent way: black and white are inverted. The patterns were also cleverly chosen such that scanning the barcode upside-down does not confuse the scanner.
In MARC, UPCs are recorded in the 024 field with first indicator 1:
024 1_ ǂa 075993844320
In OCLC, they can be searched using the sn: (Standard number) index.