What does "arms and the man" mean?
The title is an allusion to the first line of Vergil's Aeneid. In English, the line translates as "I sing of arms and the man." In Vergil, "the man" is, of course, Aeneas, and "arms" refers to the Trojan War & Aeneas' journey from Greece.
Because Shaw's play is a satire, the title should be looked at ironically. Rather than praising "arms" & the men who use them, Shaw is dissecting the reality of war, showing the futile nature of taking up arms. The characters in Shaw's play, especially Major Sergius Saranoff, serve to underscore the traditional heroism in war of the epic. Saranoff becomes a caricature, desperately clinging to his romanticized ideal of a hero. He struggles to be defined as one himself, but Shaw uses the character to instead suggest that no man could compare to a mythological hero in reality.
Arms and the Man presents a world where the practical man who lives with no illusions and no poetic views about either love or war is shown to be the superior creature. The author also tried to described the important social issue of class during that time.