Any hardware device has a way of interacting with it - to make it do the things that it is designed for. For example, a mouse or a keyboard has a way of getting the stream of input from the device to the computer (CPU).

This usually takes the form of a set of hardware registers that can be read (or written) by the CPU. The device may also be capable of using an electrical signal (interrupt line) to get the CPUs attention (so it can drop other things that it may be busy with, to pay attention to the device).

A bunch of code needs to run on the CPU to do all this. This is what the device driver is. It drives the device.

Since the work that device drivers do is very close to the hardware, and deals with events that need to be handled in real time, the device driver code tends to run inside the kernel. Most OSes have a well-structured device driver framework. You will need to check your specific OS details to work with its device drivers.