Something is conceptual when it deals primarily with abstract or original thoughts. A conceptual plan is one in an early stage. To make it work, you'll need to flesh out the details.
Conceptual art is art in which the concept seems more important than the physical aspects of the artwork. In New York's Museum of Modern Art, you can see Marcel DuChamp's conceptual piece, "Bicycle Wheel," which is a bicycle wheel mounted on a stool. Why is this art? The concept, or idea, behind it is that everyday objects become art when looked at outside of their uses. In general, when something is conceptual it takes a bit of thought to figure it out.