A slot back is basically a slot receiver who lines up in the backfield.
This page is pretty instructional.
"The flanker can also become a slot receiver or slot back. If he’s positioned between the split end and a tackle, his name changes. The coach can take out a tight end, making a slot back the third receiver, attempting to create mismatches with the defense. But even in a standard set that includes a tight end, the receiver can line up between the split end and the tackle and be called a slot back. This gives him a few steps running start before the defender can smack him one."
Wikipedia says a slot back is:
"A receiver lining up in the offensive back field. Canadian and Arena football allow them to take a running start at the line. They are usually larger players as they need to make catches over the middle. In American football slot backs are typically used in flexbone or other triple option offenses while Canadian football uses them in almost all formations."