Mending is a line manipulation technique. The point of mending is to extend the drag free portion of a dead drift. It is extended by by taking the down stream bow in the line and by rolling the rod tip up and over (mending) the the bow no longer pulls on the fly (drag free). It easier that it sounds but does take some practice.

See Presenting the Fly, by Lefty Kreh, page 190-191 or any other fly casting reference. The picture make pretty clear whats happening. Then go try it on a river and see how it really works! That's the best part.

Enjoy!

Joe