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.