Okay kiddo, imagine you have three friends, let's call them John, Jane, and Tom. You want to make a list of things you all like to do together. If you don't use a serial comma, your list might look like this: "We like to play games, swim and hike." But, if you use a serial comma, it would look like this: "We like to play games, swim, and hike." The serial comma is the little comma that comes right after the word "swim."
Why is the serial comma important? Well, it helps us avoid confusion. Without the serial comma, the sentence could be read as if "swim and hike" were one activity, like you're swimming uphill or something. It's always a good idea to use the serial comma to make things clear for everyone who reads it.