This is a good article about the meaning of that line...and an interesting tidbit about its 'fifth verse':
Oliver Wendell Holmes Sr., in 1861 added a “fifth verse” in support of the Union and against slavery:
When our land is illum’d with Liberty’s smile,
If a foe from within strike a blow at her glory,
Down, down, with the traitor that dares to defile
The flag of her stars and the page of her story!
By the millions unchain’d who our birthright have gained
We will keep her bright blazon forever unstained!
And the Star-Spangled Banner in triumph shall wave
While the land of the free is the home of the brave.
This was not an “official” verse of the national anthem, but was printed in songbooks during the Civil War.
https://ijr.com/2016/08/684228-natio...y-bad-defense/