The Oxford Comma is the comma that is sometimes used before the word "and" to create greater clarity. For example, "I like ham and eggs" indicates you like the dish: ham and eggs, but "I like ham, and eggs", indicates you like both ham and eggs, separately or together.
Unfortunately, many English speakers, including teachers, think the Oxford comma is an outdated concept and that there should be no comma before "and", ever.
Below is a link showing how the absence of an Oxford comma cost one company $5 million dollars US.
Without the Oxford (serial) comma, the following sentence has an unintended meaning:
I love my parents, Barack Obama and Michelle Obama. [my parents are Barack and Michelle Obama]
But.... my parents are not Barack and Michelle Obama.
However, if I use the Oxford comma, the sentence shows us that there are 3 people I love:
I love my parents, [AND] Barack Obama, [AND] Michelle Obama. [A, B, and C]