What does by the dawns early light mean?

It's a question. "Can you see [something; in this case, the flag] by the dawn's early light" means "can you see by the first light of dawn (or, by the first light of daybreak)." The first part of that question reads thus: "O! say can you see by the dawn's early light

What so proudly we hailed at the twilight's last gleaming" Stated in other Words, what that is asking is, "Hey, in the first light of day, can you still see what we were looking at so proudly last night when the sun set?"