A device just means any kind of component that's part of or attached to your computer. It can be located inside or outside the computer. A mouse, for example, is a device that sits outside the computer, while an internal disk drive (another device) is inside the computer. Devices need instructions on how to communicate with the printer or the rest of the computer, and sometimes those instructions aren't part of the computer's standard operating system.
In this situation, you have to add a little piece of software (a file) called a device driver. The term device always refers to hardware, as contrasted with device driver, which always refers to software. In DOS ,the word "device" is used as shorthand for "device driver" in the CONFIG.SYS file (CONFIG.SYS sets up your system when you start the computer). To activate a DOS device driver, you add a line such as any of these to your CONFIG.SYS file:
DEVICE=CDROM.SYS (CDROM.SYS is the device driver for a CD-ROM player)
DEVICE=VIDDRVR.SYS (VIDDRVR.SY is the device driver for a non-standard, large-screen monitor)
DEVICE=HIMEM.SYS (HIMEM.SYS is an extended memory manager)