what does the heredity of 50% mean? How can one translate this number into some useful and operational theories?

No matter whether it is a broad heredity or narrow heredity or something else, a heredity of 50% just says genetic variation accounts 50% of total variation (for the broad heredity) at the population level. It may suggest strong or weak hereditary. An example is the heredity of IQ. It is said, also from Minnesota Twin Family Study, that the heredity of IQ may be as high as 70%. But this won't answer whether we can change IQ or not, and whether the racial difference in IQ is due to genes or environment.