Blow-by occurs when the piston ring fails to keep the burning exhaust gasses from leaking down past the ring. Burning exhaust gasses are hot, and in addition to losing power because those gasses are leaking past the ring, robbing you of combustion pressure, the hot, burning gasses also have a tendency to overheat the piston. Additionally, exhaust gasses leaking past the ring on single ring pistons will mix with the fresh intake charge behind the piston, diluting it, and further robbing you of power when that diluted charge is delivered to the combustion chamber via the transfer ports.
Pistons are designed so that the piston crown absorbs as much of the heat as possible from the burning gasses. This heat is then transferred through the rings to the cylinder wall. From the cylinder wall, the heat travels to the outside surface of your cylinder where the cooling fins are located, and it is whisked away by the air rushing past your cylinder. This is how you keep your piston cool. If the piston isn't transferring heat away from itself, it will expand too much, and bind in the cylinder, probably causing a seizure. Blow-by allows heat to leak past the piston rings, and down to the body and skirt of the piston, where it cannot transfer heat to the cylinder wall as easily. Heat will accumulate, and lead to decreased performance, and possible seizure.
You can check for blow-by by taking your top end off. If the piston has black or brown discoloration, or carbon build up below the piston rings, it is experiencing blow-by. Some top ends run fine with a a little blow-by, but it is a condition you want to eliminate if at all possible. In addition to a properly sized piston ring gap, making sure that the piston ring grooves and ring lands are free of all carbon build-up is a good preventative measure.