No such thing as a stupid question, I didn't know either!
A circle journal is a group of scrappers who decide on a theme. You make your own book with a few scrapped pages and some guidelines (color, theme, sign-in, etc), then send it to the next person in the circle (you always send the next one in line to the same person). I have one going now that my theme was Wishes (either the fireworks or wishes come true or made while at WDW). The next person in my circle will scrap 2 pages using her photos/journaling, then she will send mine on to the next person and so on. Each month, you get another person's book (goes around in a circle) until your book comes back to you, full of everyone's pages about your theme. It's a fun way to see other's styles and just share a bit about your experiences with other people.