Inulin is classified as a dietary fiber. It's an indigestible polysaccharide that ends up being eaten by microbes in your gut.
A lot of packaged foods that are high in fiber that you wouldn't expect to be, have inulin. I remember buying some plain 'greek' yogurt for a while that was advertising fiber per serving, and it had added inulin. Anything containing chicory or jerusalem artichoke is probably high in inulin.
If you're not used to eating it - and this is true of a lot of dietary fibers and prebiotics - be prepared for gas. Like potentially uncomfortably large amounts of it. That side effect is supposed to lessen as your gut adjusts to the change, though.
I take one every other day.