A comma is a very important punctuation mark and the most common way to separate one list item from another. It is also an aid to reading as it indicates a brief pause.
Head along to your dashboard to learn about Commas in Lists with Avalon. She'll give examples of how they are used, and will also explain how the final two items in a list are usually separated by 'and' or 'or'.
Why not make a list of the colours in this rainbow with your child and see how many commas you use?