Answer:
The word "spangled" is an old word that is not used much in conversation these days, except in the National Anthem. It meant a small and shiny object, often sewn onto fabric to use as a decoration. Probably, Francis Scott Key thought the stars in the flag reminded him of the stars in the sky, because the stars were glittering and beautiful; and seeing the flag still waving after a difficult battle in the war, was also beautiful to him.