What does Genesis 7:12 mean?

The previous verse describes the two directions the floodwaters came from. On the one hand, water gushed ("burst forth") from below the surface of the earth. On the other, a torrential rain poured down out of the sky. Now we're told for the first time that it rained for 40 days and nights.

The coming chapters will explain that 40 days of that kind of rain, combined with the water uncorked and bursting from below, will be adequate to cover the entire earth. Once again, the earth will be covered with water as it was before the second day of creation (Genesis 1:9–10). As with prior verses, scholars note that the term 'erets is used here, translated as "earth," which is often used in a symbolic or local sense. The Hebrew term tebel, specifically meaning the entire planet, is not used in the flood accounts. This is part of the debate over whether the flood was planetary, or only in the region occupied by men.

Regardless of that question, there is no question that this torrent of water accomplished its purpose: the elimination of the entire human race, except for Noah and his family.