50% of what? (which is to say, you picked that number because 50% genetics 50% environment feels like a good split, and how do you even measure that?)

It means that if you plotted a graph of orphaned children IQs on the X axis and their biological parents IQs on the Y axis you'll get a messy and very bulgy line going up and to the right.

And this kind of thing is discovered by noticing that the IQs of identical twins are much more similar than those of non-identical twins.

Wikipedia has a fairly good overview: https://en.m.wikipedia.org/wiki/Heritability_of_IQ