There are several hints in the game's strategy guide. I would suggest reading that.
To add though:
-H/9 interacts with the contact rating to determine hit quality
-K/9 interacts with the vision rating to determine whiff likelehood. If k/9 is low, you'll see more foul balls rather than whiffs, and more balls in play in general
-HR/9 used to interact with power during games, but this only happens in the sim engine now.
-The two clutch ratings come into play when there are men on base/men in scoring position.
-Stamina acts as a modifier for the per 9 ratings i.e. tired pitchers become less effective, which changes the batter-pitcher dynamic over the course of the game.