My father came from such a church.
It’s an old term, from around 1800-1850. It refers to a church that was strictly abolitionist (anti-slavery), as opposed to the methodists who felt that the Bible approved of owning slaves. Those churches eventually split off into the Southern Methodists.
Around a hundred years later (WW II time) most of them joined back together. But some churches (mostly in the deep south) thought the combined churches were not conservative enough, and especially, not segregated enough, so they remained as Southern Methodist congregations.