Blade tracking is a term used to describe whether the tips of the rotor blades on a helicopter are at the same height.
Blade tracking is important because if the blades are at different heights, the dynamic balance will be off. The helicopter will want to "wiggle" with every rotation of the rotor. This causes vibration problems.
Blade tracking is checked by hovering with the rotor disc at eye level. (Be sure to keep a safe distance!) If the tracking is off, the rotor tips can be seen to be at different heights. Sticking different colour stickers/tape onto the rotor tips makes it possible to tell which blade is high and which blade is low. When the blades appear level, the tracking is correct.
Poor tracking is caused by differences in the angle of attack and shape of individual blades, due to manufacturing tolerances in the blades and grips and also any differences in linkage lengths.
Tracking is adjusted on a CP helicopter by adjusting the lengths of the pitch links in the head. It is best to adjust the pitch of one blade only (two rotor head) to avoid chasing the problem between the blades and sending the neutral pitch setting off. Use full turns only as most ball links are designed to fit the balls a particular way round.
On a FP helicopter, tracking can often only be adjusted by manually twisting plastic blades to alter their angle of attack at the tip. With wooden blades, this is not practical, so tracking is adjusted by placing asymetrical shims, perhaps made from sticky tape, between the blade grip and the blade root.