It means “will” or “must.” It is used much more in British English than in American English: Americans almost never speak the word. It is often used in legal documents and instructions to mean “you will/must do this” or “this will/must happen” (e.g., “This contract shall expire on 31 December, 2018”). When used in questions, it often replaces “should” or “do you want to” (e.g., “Shall we go now?”).

Often in engineering requirements documents, "shall" means that the requirement is absolute, that you must do it. Otherwise, a recommendation is denoted by the term "should". See e.g. http://www.ietf.org/rfc/rfc2119.txt.