What does title of "Bishop" mean in some African-American churches?

I'm familiar with what "bishop" typically means in mainline, churches. It specifically points to an overseer of several other pastors and/or their congregations. Some African-American Christian religious figures, or leaders of "historically black churches", use the title "Bishop", but in my estimation, they may be leaders of a congregation, but not overseers of other congregations. Q: What does the term mean in these contexts, and how is the term derived?