Kumbayah is a Gullah pronunciation of the phrase "Come by here" (or "come by yah", 'yah' being a West African form of 'here'.)
This word has been turned into an ironic term by overusage and misinterpretation by the mainstream culture. However, it's actually a call to God to visit the singer. "Kumbayah my lord" means 'come by here, my lord".
"Let's all join hands and sing Kumbayah."