Wing in Ground Effect is basically a phenomon that causes the plane to float on landing making it difficult to land on a spot.
Its caused by the air being 'sandwiched' between the bottom of the wing and the ground increasing the lift of the wing, and therefore making it difficult to put down. In reality its a bit more complex than that, as the low altitude reduces some of the drag on the wing but the above is a common description that most people understand.
With the size of the wing on the Canberra I think Matt is saying that it would be a fun plane to land on a carrier where you have to get it right first time.
When gliding I have used it in an emergency to keep flying when I have been short of a safe landing zone. By flying low enough (around 5-10 ft) and fast enough, you can stay in the air for a suprising distance. Obviously its something that you need training and practice to do safely but its fun.