They are with Saul and all the men of Israel in the Valley of Elah, fighting against the Philistines.”
Forty days the two armies lay encamped facing one another, each advantageously posted, but neither forward to engage. Either they were parleying and treating of an accommodation or they were waiting for recruits; and perhaps there were frequent skirmishes between small detached parties.
Commenting on 1 Samuel 17:12-30
Now Saul, and they,.... That is, the sons of Jesse, and brethren of David: and all the men of Israel; the soldiers in the army: were in the valley Elah; or "by" it, near unto it; for they were set in array on the mountain on the side of it: fighting with the Philistines; not actually engaged in battle, but drawn up for it; prepared...
Fighting with the Philistines - See at the end of the chapter, Sa1 17:58 (note).