A driver is ... well ... a driver.
The driver stack is when drivers connect to each other, each of which adds a bit to the chain. Like, say, OpenGL or DirectX drivers that use your video card driver, which in turn uses PCI Express or AGP or other drivers to configure resources and click pixels, etc. Or your USB device drivers, which, of course, must use USB drivers for communication.
It is technically possible to have a "stack" consisting of one driver, but when people say "stack", they almost always mean more than one. Or they are just trying to sound like they know what they are talking about.