Okay kiddo, so have you ever heard of a font? It's a type of lettering that we use on computers and in books to make things look nice and easy to read. Well, one type of font is called serif.
When you look at each letter in a serif font, you'll notice there are little tails coming off the ends of the lines. It's kind of like putting a little hat on each letter! For example, look at these two letters: A and B.
A in serif font looks like this: A
B in serif font looks like this: B
See the little tails on each letter? That's what we call the serif!
People have been using serif fonts for a really long time, like hundreds of years. They're especially popular in things like books, newspapers, and magazines, because they make the text look more formal and traditional.
So in summary, serif is a type of font that has little tails on the ends of the letters. It's been around for a long time and is often used to make text look more formal.