What is that that "???" in the upper right corner?

Hint 1 of 5:

I think you're going to need to shed some light on this problem.

Hint 2 of 5:

There's only one source of light on this screen.

Hint 3 of 5:

Use the lantern to turn it on (F).

Hint 4 of 5:

You really want to catch the lamp fish that swims by.