Originally Posted by LouW

Roll coupling is just another name for lateral stability. It is the tendency to roll away from a sideslip. In normal flight this is generally desirable since when a wing drops, the airplane slips toward the low wing and the roll response tends to lift the low wing back to level flight. As others have described, it is roll in response to yaw.

In case of knife-edge flight, the airplane is in a constant yaw (sideslip) and any roll response must be countered by control input or the airplane would roll back level.

As a general rule, for aerobatic flight, stability about all three axis should be a minimum, whereas for normal sport flying a greater degree of stability results in easier and more relaxing flight. The desired degree of stability is very much a personal preference and can be designed in to be almost anything the designer wants.