What does the 30 30 mean in ammunition?

.30 caliber bullet, 30 grains of blackpowder when it was originally made. Smokeless powder is now used.

Actually the 30-30 began life as the 30 WCF (Winchester Center Fire) aka 30 Winchester Smokeless or 30 SMOKELESS. The second 30 refers to 30 grains of smokeless powder. (30 grains of black powder would have been a pretty whimpy load)