Question

I don't understand what an oxford comma is and how it's used can you help me

Answer 1

See here--> http://grammar.about.com/od/grammarfaq/f/QAoxfordcomma.htm

Answer 2

An Oxford comma is a series comma. If you have 3 or more things listed, then it's the comma before the last item.

For example:

I drew a picture of a boy, a girl(,) and a dog.
The Oxford comma was in parenthesis.

In English class it's mostly necessary, but some academic snobs don't like it; if you're in college, ask your professor his or her opinion on it and just stick with that. Journalists, though, never use it.