How your Body Balances
What is your body's centre of mass?
If you were to cut yourself in half perfectly down the middle, from the top of your head to your bottom, then each half would have the same weight (unless you only have one arm or something). Now imagine this in 3 directions, front on, side on, and across the middle.
The Centre of Mass for your body is the point where the weight of each of the 3 planes is equal.
What is the Base of Support?
The area in contact with the object you are balancing on. The bigger the base of support, the easier it is to balance.
If you keep your centre of mass directly above the middle of your base of support it is easiest to balance. When you are off centre, you need additional strength to hold the position. The further off centre, the more strength required.