Meaning of "climactic battle"

“…the hero strings his bow for the climactic battle…”

What does “string one’s bow” mean? And what is a “climactic battle”?

To string one’s bow is to place the bowstring on the bow, to prepare that weapon for battle. A bow when not in use does not usually have its string mounted-- that would weaken the bow.

A climactic battle is one which is the climax, the deciding battle of the war (or the movie).