This verse from 1 Samuel 17:21, "Israel and the Philistines put the battle in array, army against army," simply means that both the Israelite and Philistine armies formed their battle lines, ready to engage in combat.

Here's a breakdown of the key phrases:

"Israel and the Philistines": Refers to the two opposing armies in the conflict.
"put the battle in array": This is an older, more formal way of saying "prepared for battle" or "formed their battle lines." "Array" in this context refers to the organized formation of troops ready for combat.
"army against army": Emphasizes that the two fully formed armies were facing each other, poised to begin fighting.

In essence, the verse is setting the scene, describing the armies getting into position and creating a sense of anticipation before the actual battle begins. The phrase encapsulates the tension and readiness before conflict erupts.