Hardware is best described as a device that is physically connected to your computer or something that can be physically touched. A CD-ROM, monitor, and printer are all examples of computer hardware.
How hardware can cause issues
Computer hardware requires resources to correctly operate with the computer and the software installed in the computer. If your computer does not have the available resources, such as available IRQs, this will cause your computer to not properly work and/or the devices to not function properly.
In addition to needing resources, not all computer hardware will work properly with all computers, or may have incompatibilities with hardware that is already installed in your computer.
Finally, almost all computer hardware requires some type of software or drivers to be installed before it can properly communicate with the computer. If these drivers contain problems or do not properly work with the computer, this can cause issues with the hardware device you are attempting to install in the computer.