Bullet Drop Compensation (BDC) is a feature available on some rifle scopes. The feature compensates for the effect of gravity on the bullet at given distances. The feature must be tuned for the particular rifle and cartridge.
Bullet Drop Compensation is also available to accept changes in wind and elevation. There is a dial on most scopes that allows the user to turn it by single "clicks" that adjust for the current situation. Each scope is different and requires different adjustments. (MASC151)