Take a look at this sentence:
"My dog is lazy, chubby, and old.
Did you notice the commas?
We use commas when we're listing more than two things. There is a comma after the word "lazy" and before the "and" to show the separation of the three items.
The Oxford comma, also known as the serial comma or the Harvard comma, is the comma you see before the "and" and "or". This comma makes it easier for people to separate the items you are listing and it also avoids ambiguity.
In a sentence with a simple list like the example above, the meaning is clear and most people will immediately know what you mean. But there are some cases where it can get confusing. Take a look at this example:
If we add the Oxford comma before the "and," the sentence becomes clear that you mean you want to thank (1) your parents (2) Bill Clinton (3) Lady Gaga. It won't mean that your parents=Lady Gaga and Bill Clinton.
Here is another example of how the Oxford comma can help eliminate confusion.
"Every morning I have a cup of coffee, sausage and eggs and toast."
This is very confusing. Does it mean you have:
(1) coffee (2) sausage (3) eggs and toast
(1)Coffee (2) sausage and eggs (3) toast
If we introduce the Oxford comma, the meaning becomes clear:
"Every morning I have a cup of coffee, sausage and eggs, and toast"
Every morning you have: (1) a cup of coffee (2) sausage and eggs (3) toast
"Last night I met Trish, a baker, and Margaret "
Did you meet three people or is Trish a baker? In cases like this I would suggest rewording a sentence instead of listing so it's clearer.