Who or what is the Driver?

I assume that you are asking this question in the context
of slavery.  If so, the drivers, or slave drivers, were people whose job it was to make
the slaves work on a moment-to-moment basis.  In other words, the drivers could be seen
as the lowest level of supervisor or management for the slaves.  They were the ones who
were watching the slaves do the work and forcing them to work hard enough to suit the
upper levels of management.

On a large plantation, there
was a real need for drivers.  A plantation owner with, say, 100 slaves could obviously
not keep an eye on every slave's work.  Neither could his overseer.  Therefore, there
was a need for drivers to supervise smaller groups of slaves.  These drivers were either
white people or were slaves themselves.