The songs were spontaneous, passionate, and moving. The slaves would sometimes "sing the most pathetic sentiment in the most rapturous tone, and the most rapturous sentiment in the most pathetic tone." Douglass believed the hearing of those songs would do more to impart the terrors of slavery than reading volumes of philosophy on the topic.
When he was a slave he did not truly understand the meaning of the songs; they were "beyond my feeble comprehension; they were tones loud, long, and deep; they breathed the power and complaint of souls boiling over with the bitterest anguish." The songs were what truly allowed him to glimpse how dehumanizing and miserable slavery really was.
Douglass commented that he was astonished how people claimed that slaves sang because they were happy, and thus concluded that they were a happy bunch of people. A slave's woes can only be relieved by singing. It did not make sense to him to sing for happiness, only for misery.