The Oxford Comma is the comma that you find in a list of several items, just before “and (last item).”
For example: Yesterday I worked in the kitchen, made phone calls, (Oxford Comma) and did a lot of filing.
The simplest reason to use an Oxford Comma (aka. serial comma) is that it prevents confusion and ambiguity. Sometimes it isn’t necessary, like in a list of single words, or words that are related.
Today I am wearing jeans, socks and a sweater. (without Oxford Comma)
Today I am wearing jeans, socks, and a sweater. (with Oxford Comma)
Whether I use the Oxford Comma or not doesn’t change the meaning of the sentence.
However, in certain types of sentences, the non-use of an Oxford Comma can lead to some rather confusing (and hilarious) interpretations.