First, the Basics:
[j]ump to some maildir: after pressing j (“jump”),
mu4e asks you for a maildir to visit. These are the maildirs you
set in Basic configuration and any of your own. If you choose
o (“other”) or /, you can choose from all maildirs under
the root-maildir. After choosing a maildir, the messages in that
maildir are listed, in the The headers view.
enter a [s]earch query: after pressing s, mu4e asks
you for a search query, and after entering one, shows the results in the
The headers view.
[C]ompose a new message: after pressing C, you are dropped in
the Composer to write a new message.