The Oxford comma is the last comma we use when we’re writing a list of things. Check it out:

→ I’m gonna buy juice, milk, bread, and apples.
It’s very important to use it when you’re writing formally, like an email at work or a letter to your university, or to make yourself clear in certain situations. For example:

→ I had pie, milk, cake and jelly for breakfast.
If you ate the cake with the jelly together, that’s great! You wrote it right. But if you ate them separately, you should use the Oxford comma:

→ I had pie, milk, cake, and jelly for breakfast.
Now, you made yourself completely clear!

Another example would be:

→ I love my sisters, Ariana Grande, and Adele.

This comma shows that you love different people. But, if you remove that last comma, everyone will understand that your sisters are Ariana Grande and Adele (which would be awesome but improbable).
I hope you liked this lesson. See you soon!