An open-jaw flight is when you fly from Point A to Point B and then Point C to Point A, or from Point A to Point B and then from Point B to Point C. For example, you might fly to London from Seattle, but then fly back from London to Portland. Or you might fly from NYC to Paris, but then return to NYC from London
You’d book it the same way as a multi-city flight and can be useful if you plan to do a lot of on-the-ground travel.