A modifier key is a key on a computer's keyboard that is only used in conjunction with another key. Modifier keys are often used with other keys to perform computer keyboard shortcuts and other commands. For example, in many text editor programs pressing the Ctrl+A will select all text.
Examples of modifier keys
Additionally, most laptop and some desktop keyboards contain an Fn modifier key.
Tip: If you have difficulties pressing and holding a modifier key while pressing other keys you can enable the StickyKeys option in Windows.