The Oxford Comma is the last comma before the final item in a list, for instance: dogs, cats, birds, and fish. The comma before “and fish” is the Oxford Comma.
Now in modern American English, use of the Oxford Comma has been denigrated, meaning that it’s optional in most cases. I personally tend to eliminate it unless doing so would render the sentence harder to understand.
A dairy in Maine will now have to pay $5 million in back overtime wages because of a missing Oxford Comma in a state law, which stipulates that workers are exempt from overtime when in the process of “…marketing, storing, packing for shipment or distribution of” products.