This comma goes by many names, including the serial comma and the Harvard comma, and is an issue of quite a bit of anxiety for some. (For the purposes of this response, I’ll call it the serial comma.) It’s the comma that comes before the final item in a list of three or more:
I packed a deck of cards, pretzels, and three novels.
See that comma before “and” toward the end of the sentence? That’s the serial comma. And why so much debate? As long as the sentence is clear, you can use it or not use it at your discretion. As always, there are notable exceptions. If you’re following a style guide, look up the particulars. Chicago Manual of Style advocates for its use, while AP Stylebook recommends skipping it if the sentence is clear.
This issue of clarity is where things get sticky. In the example sentence above, the sentence is clear without the serial comma. But this next sentence changes meaning depending upon the absence or presence of the serial comma:
I mailed the invitations to my friends, the mail carrier, and the fire chief.
With the serial comma, there are more than three invitations going out: one to each friend, the mail carrier, and the fire chief. Without the comma, it reads like this:
I mailed the invitations to my friends, the mail carrier and the fire chief.
That makes just two invitations: one to the friend who is a mail carrier and the other to the friend who is a fire chief.
Take your stance on the serial comma and be consistent with it, while at the same time keeping tabs on clarity.