C-h: 'C-h S' to get help on that obscure bash built-in or elisp function, 'C-h v' to get help on an elisp variable (most modes offer a lot of customisation vi

C-h: 'C-h S' to get help on that obscure bash built-in or elisp function, 'C-h v' to get help on an elisp variable (most modes offer a lot of customisation via setting particular variables), 'C-h m' to show the current mode's keybindings, 'C-h k' to find the name of the function bound to a particular key sequence, 'C-h t' for a tutorial on all the basic (and not so basic) editing & buffer/window management commands, and 'C-h ?' to get a list of other help commands.