C-h a (a is for Apropos.)Then do
file
C-x oto go to the other half of your (now split) window, and then
C-v (to move down it, just like usual)and you find:
insert-file C-x i Function: Insert contents of file FILENAME into buffer after point.
C-h k (k is for Keystroke.)This yields the command save-buffers, and a paragraph or so explaining how to use the command. This is also useful because you can then execute the same command by typing:
C-x C-s (and then the return key)
M-x the-command-nameIn this case, it would be
M-x save-buffersOr, you can bind the command, possibly with another set of commands, to a function key. (See the stock answer How to use the FUNCTION keys in Emacs.)
C-h ? C-h (h is for help. So is the '?'.)gives a list of the different kinds of help that emacs has, and how to use them. The most expansive of these is the Info browser, invoked by the key sequence C-h i. See the stock answer "Using the INFO browser" for details.
C-h m (m is for mode)This gives help that is specific to to the mode you're in, such as Latex or Mail. It usually gives a brief description of the mode, as well as the additional commands specific to that mode.