Well, you're in luck: I just happen to have a book right here that tells us.
The function of the rays (called medullary rays) is to store and transport food throughout the tree. They radiate from the pith area towards the outside of the trunk. "The medulllary rays in oak can be seen on the face as dark brown lines about one inch long"
As to the question, how do they affect bowmaking? I think the answer is simply, they create an attractive appearance (if you like the look of them) or they make the bow look ugly (if you don't like the look). I have made a white oak bow before, and found the rays to be no impediment at all: just looks good.
