What does Judges 11:17 mean?

This is part of a response sent by Jephthah, through messengers, to the king of Ammon (Judges 11:12–14). He is explaining how and why the Ammonite ruler is wrong about the people of Israel improperly taking Gilead during the time of Moses. He has described how Israel came to be camped at Kadesh, south and west of the Dead Sea (Judges 11:15–16).

Eventually, the time came for Israel to travel to the east side of the Jordan River and cross over into Canaan. The quickest way for the Israelites to get to the right place would have been through the territories of the Edomites and the Moabites. However, the Lord had been clear that they were not to fight with either nation. The Edomites were the descendants of Isaac's son Esau, and their land was given to them by God (Deuteronomy 2:4–6). In the same way, the Moabites were the descendants of Abraham's nephew Lot and they also possessed land set aside for them by God (Deuteronomy 2:9).

So, the Israelites sent messengers to the king of Edom, respectfully asking to pass through his land. They offered to pay for any resources they used along the way. The king forcefully refused (Numbers 20:14–21). The king of Moab refused, as well. To obey God, Israel was forced to linger in Kadesh and take the long way around to reach their destination.

Jephthah's point to the king of Ammon is that Israel did not simply take land. They sought nothing which was not given to them by the Lord, as Ammon was accusing them.