The modifier keys let you to use the keyboard to produce more possible signals than there are keys. Just as the shift key lets you use 26 keys (a–z) to produce 2×26=52 characters (a–z and A—Z), so the other modifiers let you send a lot of different signals. Modifier keys can be combined (for example, shift and control and A).
The meanings of modifier keys can be mapped to anything, but here are the conventional uses. The symbol shows you how these usually appear in documentation. Some keys can be written in more than one way (for example, the Control key may be “Control”, “Ctrl”, “Ctl” or sometimes “^”).
|⇧||shift||⇧||shift to upper case|
|generate alternate character (e.g., £ or #)|
|turn the other key(s) into a control signal|
|fn||function||Fn||turn the other key(s) into a function|
|specific actions — see more about the Windows key|
|execute shortcut (on Mac only)|
If you see ctrl+shift+C (or ctrl-shift-C), that means “hold down the control key and the shift key and then press C”. Those first two are modifying the C. What that actually does will depend on the program that is listening to it.