It's just covarience with genes -- this might seem like it means "X is genetic" -- but it *doesnt*. Consider accents (eg., the scottish accent) this is highly heritable: typically only people with the genetic profiles of those localised to scotland have it.
Consider using heritability to determine genetic vs. envrionmental causation (largely: you just cant use it to do this). Eg., even in the case of height: it would seem that the genes of people in poor-nutrition countries were the cause of their low height.
The movement from "heritability" to "genetic causation" *asssumes* (and always, by assertion *assume*) that the trait studied is already genetically caused. IF we know this, THEN hertiability is a somewhat reliable measure of "to what degree".
But in almost all cases we have no grounds whatsoever to independently assume this. Rending most heritability studies basically useless.