What does Judges 1:35 mean?

As was the case with the other tribes of Israel (Judges 1:27–34), the people of Dan have not been able to drive the local inhabitants out of their territory along the coast of the Mediterranean. Instead, the Amorites there have been able to drive the citizens of Dan off the plain and up into the hill country, where they settle.

The Amorites remained entrenched in Mount Heres, perhaps also known as "Sun Mountain." They are also secured at Shaalbim and Aijalon, a city guarding an important trade route in the region.

The phrase "the hose of Joseph" refers to the tribe of Ephraim (Genesis 48:3–6). Since the places described in this verse were so near to Ephraim's territory, that tribe apparently became strong enough to eventually cross into Dan's territory and enslave Amorites of that region. It's important to recall that whenever any of the tribes enslaved inhabitants of the land, they are making a willful choice to disobey God's command. Awful as it may seem, their mission was to destroy the intensely wicked Canaanites (Deuteronomy 7:1–4; 9:4–5). The price for that repeated disobedience will be quite high (Deuteronomy 20:16–18).