A modifier key is a key on a PC’s keyboard that is just used in combination to another key. Modifier keys are regularly used with different keys to perform PC keyboard shortcuts ways and different commands. For example, in text editor programs pressing the Ctrl+A will all text content.
Some example of modifier keys
On an IBM compatible PC, modifier keys are Alt, Ctrl, Shift, and the Windows key. On the Apple Macintosh PC, the Control, Option, Command, and Shift keys are modifier keys.
Moreover, most laptop and some desktop keyboard contain an Fn modifier key.