The meaning is this:
People in a society have some ideas and impulses which they hide, perhaps because they are afraid to show these ideas, or because the ideas and impulses are taboo
So people choose to show these ideas and impulses in a less direct way, through the arts (painting, music, theatre, etc...) - perhaps they paint about an impulse or write about it in a play.
"otherwise hidden" means that they are hidden in other respects, in other ways.
So, the arts reveal, or show, these impulses and ideas, which are not shown in any other way.
Hope this helps.