The Oxford comma is the comma that comes before “and” or “or” in a list of three or more items. It's named after Oxford University Press, which requires it in their publications.

Without an Oxford comma, you might write: “I brought sandwiches, fruit and cookies.” With an Oxford comma, you write: “I brought sandwiches, fruit, and cookies.” Both are grammatically correct, but they can sometimes create different meanings.

Consider this sentence: “I'd like to thank my parents, my basketball coach and my chemistry teacher.” Without the Oxford comma, it sounds like your parents are your basketball coach and your chemistry teacher. Add the comma and it's clear: “I'd like to thank my parents, my basketball coach, and my chemistry teacher” means you're thanking four different people or groups.

Some style guides require the Oxford comma while others say to skip it. The choice often depends on where you're writing and what sounds clearest. Either way, the goal is the same: making sure readers understand exactly what you mean.