The Oxford Comma is a comma placed after the word ‘and’ at the end of a list. Typically a list will have commas separating each item until you get to the end, at which point the word ‘and’ becomes the final separator and a comma is not needed.
The Oxford Comma, named from Oxford University where it was mainly used, is optional but it does help clarify a list that it not made up of singular objects.
Example of a normal list in a sentence:
We got milk, bread, eggs and butter from the store.
Example of the optional but useful Oxford Comma:
Your t-shirt color choices are red and white, black and gold, and blue and silver.