An operating system is just a very complicated program. It has the job of organizing other programs on a computer, including sharing the computer's time, memory, hardware and other resources.
Since the operating system has to interact with the hardware on a computer system, it also has to have specific knowledge of the hardware on a system. To allow operating systems to be used on a variety of computers, the concept of drivers was invented.
Drivers are small bits of code that can be added and removed from the operating system in order to allow the operating system to talk to a particular piece of hardware.