SG is the specific gravity, or the measure of gyroscopic stability applied to the bullet by spin.
Mathematically SG is the ratio of the stabilizing influences of rotating mass, vs. the de-stabilizing effects of aerodynamics. If this ratio is greater than 1.000, it means the bullet has more stabilizing influence than de-stabilizing influence, so it’s said to be stable. In practice, a bullet needs an SG of 1.5 or greater to be well stabilized, and fly with the maximum effective BC.
Read our article on Stability and BC for more information on how this can effect the BC of the bullets you shoot.