Humans are not to look directly in to the sun. What will happen if one does?

This afternoon, while lowering my sun screen, the sun was (almost) in my eyes.
I wore sunglasses and was able to look in it.
Without them I couldn’t.
Then I thought about this question.
What will happen when one does look directly in the sun?
What I would like to know is what factually happens, biologicially, physiologically if that makes sense?.