The help commands to get information about a key sequence are C-h c and C-h k. C-h c key displays in the echo area the name of the command that key is bound to. For example, C-h c C-f displays ‘forward-char’. Since command names are chosen to describe what the commands do, this gives you a very brief description of what key does.
C-h k key is similar but gives more information: it displays the documentation string of the command as well as its name. It displays this information in a window, since it may not fit in the echo area.
To find the documentation of a key sequence key, type C-h K key. This displays the appropriate manual section which contains the documentation of key.
C-h c, C-h k and C-h K work for any sort of key sequences, including function keys, menus, and mouse events. For instance, after C-h k you can select a menu item from the menu bar, to view the documentation string of the command it runs.
C-h w command <RET> lists the keys that are bound to
command. It displays the list in the echo area. If it says the
command is not on any key, that means you must use M-x to run
it. C-h w runs the command