You’ve likely seen one of the various memes addressing comma confusion. A popular example demonstrates the difference between eating eggs, toast, and orange juice versus eating eggs, toast and orange juice which could conjure images of orange juice-covered toast. The former version uses the Oxford, or serial, comma for clarity, but doing so is not a rule; it is a style choice.
As in the example above, the Oxford comma comes before the conjunction in a series of three or more elements. The Chicago Manual of Style strongly advocates for using this style, while the AP Stylebook does not require it unless omitting it would reasonably cause confusion. For example, writing “I put a book, pencil and pen in my bag” is clear without placing a comma before “and.”
I personally prefer to use the Oxford comma unless a client prescribes to a style guide that says otherwise. For me, always using it is a simple way to ensure that readers will not stumble over the sentence or be confused. Others prefer to cut commas wherever they can without changing meaning. This is also an acceptable choice. Whichever style you choose, the most important thing is to maintain consistency throughout the work in question.