Mostly it seems to be quick and adaptable thinking, if I understand the test results right. The colloquial usage is all over the place, as usual, but the tests strongly correlate with things like being able to attain advanced education and ability to be successful. Some of it appears to be inherent, whether it's clock speed/RAM equivalent for our brains or what, because it survives twins raised in completely different homes (including large economic differences). That said, there are certainly environmental effects, too, especially in the negative direction (e.g. lead exposure).