The poem "Barbara Frietchie" tells the story of an old woman in a Union town during the Civil War named Barbara Frietchie. She defended the Union flag against General Jackson and so he let it be.
The action described in those lines describes a flag that kept going up and down with the word "ever" meant to describe that the movement was consistent.
The following line then gives the reason why it kept going up and down to be the wind. Another way to describe those lines therefore is to say that the flag went up and down in the wind.
