It is not clear exactly what the Philistines meant with their invitation.1 The Hebrew literally says show you a thing.
Quite possibly it was a challenge to some kind of duel, or perhaps an invitation to join the Philistines as mercenaries.2 According to 1 Samuel 14:21, there were some Israelites who actually joined the Philistine army.
12 And the men of the garrison hailed Jonathan and his armor-bearer and said, “Come up to us, and we will show you a thing.” And Jonathan said to his armor-bearer, “Come up after me, for the LORD has given them into the hand of Israel.”