Extension Jamb: Lumber extending from a window or door frame to accommodate different wall thicknesses.
Why do you need an extension jamb? Since the energy crisis of the 1970s brought on the demand for thicker insulation, most contemporary houses are framed with 2×6 studs. Windows, though, are generally still configured for 2×4 walls, so the jambs have to be extended to bring the window flush with the drywall. Fine Homebuilding Magazine has an extremely detailed article on the process for properly measuring and installing jamb extensions- you can check it out here.