Commas separate ideas or nouns (as in a list).
He’s bundled in a wool coat, mitts, hat, scarf and snow-boots because it’s so cold.
Katherine did the shopping, cleaned the house, and finished the laundry.
Durer, daVinci, and Michelangelo are all famous painters.
If you’re repeating words for emphasis, a comma will separate the repeated words so it’s clear you haven’t made a typo.
I really, really like chocolate.
Maggie has been a naughty, naughty dog.
A comma can change a noun to a verb.
The panda eats shoots and leaves.
The panda eats, shoots, and leaves.
A comma can change the person to whom you are speaking into the person about whom you are speaking, and determine the rest of the punctuation in the sentence.
Mary is the farce today.
Mary, is the farce today?
A comma can tell the reader to pause for a brief moment because what you’re about to say will add another mind-boggling idea.
The gymnast leapt several feet into the air, and then he did a triple twist and a somersault!
Commas are also used to offset information (such as found in an appositive or an introductory clause).
Yesterday, we went to the park for a picnic.
Michael, my brother, is a nice guy.