I've never been quite sure exactly what is considered idiomatic (potential new thread?), but the translation doesn't get any easier:
"one for all, all for one"
It was the slogan/pledge of the King's guards in The Three Musquateers, and has become a common expression.
It means that one is bound to serve the needs of the group. On the flip side, if one is in need (of help), he will not be left behind: all the others must assist him.
In The Three Musquateers, this also meant a commitment to die trying.