Fibonacci extension and fibonacci expansion are opposites.
A fibonacci extension goes in the same direction as an ordinary fibonacci retracement. That is, it’s an extension of the fibonacci retracement.
A fibonacci expansion goes in the direction of the original price move (before the retracement). It projects probable price levels in the event of a continuation of the original move.
Pictures will make this easier to understand. (Click the images to enlarge them.)
Note: These images are from the MT5 platform, but the treatment of fibonacci retracements, extensions, and expansions is the same in both MT4 and MT5.