Taken from here: EmacsWiki

Prefix Meaning C- (press and hold) the Control key M- the Meta key (the Alt key, on most keyboards) S- the Shift key (e.g.‘S-TAB’ means Shift Tab) DEL the Backspace key (not the Delete key). Inside Emacs, DEL is written as <backspace> . RET the Return or Enter key SPC the Space bar key ESC the Escape key TAB the TAB key

A notation such as C-M-x (or, equivalently, M-C-x) means press and hold both Control and Meta (Alt) keys while hitting the x key. From now on, I won't say something like "Press M-x" anymore. For example, if I say "C-x C-f your files", you should replace C-x C-f with its command like this in your head: " find-file your files". All commands use verbs, I think, so don't worry. Try to recall the command from the key binding; it will help you get used to Emacs quicker. One exception though, I only say "press key" if key is a single character on the keyboard.

If you see M-x command , it means you need to M-x and type command.

A prefix key is a part of a full key binding. For example, a full key binding is C-x r l to run the command bookmark-bmenu-list , then C-x and C-x r are its prefixes. Note that key sequence such as C-x and M-x are considered a single character. Knowing prefix key is handy: if you forget key bindings of some commands that use the same prefix key, and remember the prefix, you can press the prefix key and C-h to get a list of commands with that prefix.

For example, C-x r is the prefix for register and bookmark commands in Emacs. However, you forget a specific key binding for a command in those features. C-x r C-h list all key bindings that have prefix C-x r.

Finally, C-g executes the command keyboard-quit , which cancels anything Emacs is executing. If you press any key sequence wrongly, C-g to cancel that incorrectly pressed key sequence and start again.

As you gradually learn Emacs, you will see the key bindings are really systematically organized and mnemonic. Whenever you see key bindings end with n and p, it usually means next and previous; o means open; h means help; C-h is standard prefix for help commands; key bindings such as o and C-o are frequently used in many built-in tools such as Dired, Ibuffer, Occur…