A UPC is a Universal Product Code, or the series of numbers that are found in the barcode of a product. (here's a helpful article on purchasing UPCs for your products)
UPCs, EANs, ISBNs, and GTINs are all similar, and in some cases, if you do not have a UPC readily available for a product, adding an EAN, ISBN, or GTIN can help a product reach better success on marketplaces.
To add a UPC to a SKU, here is an animation that shows how:
In the Catalog, click on a product to add the UPC.
Click 'Edit' on the left side of the screen.
In 'General Info' add the UPC and save.
Having a catalogue of SKUs that are outfitted with unique UPCs is very important for success on all marketplaces, and is most important for success on Walmart.
Walmart and UPCs:
To be listed successfully on Walmart, products must have unique UPCs, even if the products are a part of a pack.
For example, let's say I have a single waterbottle for sale on my store, and a 2-pack of the same waterbottle. In this case, the single waterbottle must have it's own unique UPC, and the 2-pack must have it's own unique UPC.
Having specific UPCs is key to getting traction on Walmart especially, since Walmart requires UPCs to be different for every product/SKU.