This is an expression, used by slaves who do not wish to live
anymore, yet do not wish to commit suicide. They are hoping God
will send a chariot from heaven, and use that chariot to take them
home. It refers to the chariot that took Elijah home in the Old
Testament of the Bible.
Sweet Chariot was a coded message about the Underground Railroad.
Slaves used biblical references to provide information to the
slaves who wanted to escape such as the mode of transportation
available and how to get to different places and other important
details. Their masters thought that they were singing about God so
the slaves were able to relay the information right under their
noses. I found this out from one of my college professors who was
the grandson of a woman born a slave but was freed when she was 7.