It's worth noting that this work was originally simply a poem, and was first known as "Defense of Fort M'Henry"; later it was set to the tune mentioned above and renamed "The Star-Spangled Banner". The US Congress adopted it as the official National Anthem of the United States of America in 1931.
George Dorn was right when he said that few people know about the other 3 verses. However, if you read the lyrics expressly, you you realize that singing just the first verse is a problem -- the first verse is a question. Only in the second verse is the answer. The question we are asked is:
O Say, can you see by the dawn's early light what so proudly we hailed at the twilights last gleaming?
In a verseful way, what he is asking here is "Can you see what [the flag] we saluted last night before the sun went?", with the lines after describing the flag itself and the odd glimpse of it to be seen through the light of the cannon fire. In the last lines of the second verse, our answer is given:
Now it catches the gleam of the morning's first beam. In full glory reflected now shines on the stream; 'Tis the star-spangled banner; O long may it wave...
In the first rays of the morning sun of September 14th, after a full day of bombardment by British Forces, the soldiers marked that, although tattered and torn, it was still in place. This was a big deal, for it symbolized that the battle was not given over or lost.
History has it that Key wrote this stirring requiem the on the morning of the 14th after a long night of anxiety. Early in the A.M. of the 14th, the British were still shelling Fort McHenry, when suddenly it stopped. Key was 'below deck', and when he heard it stop he knew that could mean only one thing: Fort McHenry had been surrendered. However, what he failed to know was that General Armistead's force had sunk as many as 22 British vessels and so in mid-morning, they (British) had deemed the takeover of Fort McHenry to be too costly and had retreated.
Francis run into the early morning sun, expecting to see the Union Jack flying above his head, but when he gazed at the flagpole, Old Glory was still there in splendor, inspiring his to pen these famous lines if the back of a letter his had in his waistcoat.
Being the Amateur poet that he was, Key made several revisions of the poem, and so many early printings of it may have subtly different wording. It is also because of this that there exists two enshrined copies of the manuscript -- The original writing resides with the Maryland Historical Society, while another hand-written draft rests in the Library of Congress.