Credits: Jason Furnell
Sketchboarding is a rapid, sketch based, iterative design method for defining the blueprint of a product experience. A sketchboard shines brightest during the early stages of product design, when shared understanding is more valuable than a highly evolved design. Sketchboards are then used as an artefact of understanding, to help drive out more detailed discussions about requirement, estimates etc.
Jason Furnell has published a detailed write-up on how he uses sketchboards in the design workflow. This has everything you need to know about using sketchboards for yourself. There's even a video!