Lets look at the square to begin with. The square primarily deals with balance and stability. A square block on the ground is not going to fall over easily. Applying this to the body means that we want to remian with a straight spine the is vertically aligned with gravity and perpendicular to the ground. This will help to maintain our balance. Plus we need a stable stance that will not be moved easily. The horse stance is the embodiment of the square.
Taken to the mental level, the sqaure referrs to mental stability. One's mind is not easily moved by other forces and will remain steady under pressure.
The key words relating to the square are balance and stability. By understanding what the square is and how it functions, you can then explore how to break the square in another person.
Some quick key words for the other two shapes.
Triangle: Intention, focus
Circle: Movement, flow, continuity, coiling/uncoiling.