Hi, Building a p51 kit, unsure what counterbalances are. The kit says if u want to add them i could build extra structure in the elevator and rudder. This is my second kit build and just not sure what they are. If they are only for apperance what do they look like if anyone can help me i could use it. thank u

Counter balances are the portion of the control surface that is in front of the hinge line. If the plane has counter balances they are usually located on the outer tips of the elevator and ailerons and on the top portion of a rudder.

I'm probably using the wrong terminology here, but the counter balance is used to take some of the aerodynamic pressure off the controls. On full scale, it simply makes it easier to move the controls in flight.
I'm not sure if it is necessary on that kit. Mostly it's appearance.