Think of BC as a measure of the bullet's aerodynamic drag - i.e., a measure of how fast the bullet will slow down s it moves down range, and a measure of how flat the trajectory is with respect to bullets of the same weight, flying at the same speed, but different BC.
Since a high BC, low drag bullet loses speed slower than a low BC bullet, it arrives at the target quicker (and with a higher percentage of its muzzle velocity remaining), and therefore it is exposed to cross wind deflection for a shorter period of time than high drag bullets, so the actual deflection in a cross wind is less for high BC bullets.
Sierra measures BC in their underground tunnel. Unfortunately it's usable length is too short to gather BC data at very long ranges, over 200 yards or so (I forget the exact distance they are using). Many other shooters measure BC at regular shooting ranges using two chronographs to measure the bullet speed at the muzzle and at the target (usually at the target).
Sierra has a store at the plant where seconds can be bought by the pound, and they also give palnt tours. Starline Brass is next door.