The overall context of the struggle against slavery in the story is based on true events. The existence of abolitionists and the resistance from the slave - owning side are real historical elements. Also, some of the events that show the brutality and inhumane treatment of slaves are likely based on true accounts, although they might be dramatized for the show.
I'm not entirely sure specifically as it could refer to different things. It might be about a religious or spiritual experience related to the 'good lord' and a bird which could symbolize something like freedom, a message from the divine, or a connection to nature in a spiritual context.
Since I'm not familiar with this specific 'good lord bird story', it's hard to determine its creator. It might be the work of a single individual who was inspired by their religious beliefs and an encounter with a bird. Or it could be a collaborative effort, where different people added elements to the story as it was passed along in a community.