BDC (Bullet Drop Compensation) is a measurement for bullet drop on your reticle. BDC is very effective when you know the range of your target. In order to use the reticle properly, you will have to zero the rifle at the range that is recommended in the manual. The reticle uses hash marks, dots, or bubbles to help compensation for bullet drop and is only estimates of yardages listed in the manual. It is very important to validate the trajectory of the specific load you will be working with.