Evil souls fall from the bridge into hell.
This is a common afterlife theme found in some form or other in many cultures, such as the Chinvat Bridge of Zoroastrianism and As-Sirāt of Islam.
The "Brig o' Dread" is an important element in The Lyke-Wake Dirge, an old Northern English waking song.
Elements of the Lyke-Wake Dirge bear resemblance to concepts of the afterlife found in Germanic cosmology.
The "Brig o' Dread" is possibly related to the bridge Bifröst (which probably means "trembling-way"), spanning the divide between the world of humans and the world of gods, or the Gjallarbrú, which spans the river Gjøll ('resounding' or 'noisy', EN cogn.