John Gill Commentary 1 Samuel 19:8

"And there was war again: and David went out, and fought with the Philistines, and slew them with a great slaughter; and they fled before him." — 1 Samuel 19:8 (ASV)

And there was war again
Between Israel and the Philistines; it does not appear that either of them sent out their whole force, only some parties or detachments, between which there were skirmishes:

and David went out and fought with the Philistines ;
he went out with his thousand men, over which he was made captain, (1 Samuel 18:13) ; for he had not the command of the whole army; that belonged to Abner:

and slew them with a great slaughter, and they fled from him ;
he killed many of them in his engagements with them, and the rest fled, and he returned victorious; which stirred up the envy and increased the jealousy of Saul, to observe which this is related.