According to the author of the Book of Judges, this verse means that Moses' wife's family, called the Kenites, moved away from a place with lots of palm trees to live in a new area with the people of Judah. They went to a wild place in the south and decided to stay there with their new friends.
Judges 1:16: And the children of the Kenite, Moses' father in law, went up out of the city of palm trees with the children of Judah into the wilderness of Judah, which [lieth] in the south of Arad; and they went and dwelt among the people.
Explains that Judah could not drive out the inhabitants of the valleys because they had chariots of iron, contrasting with the idea of successful conquest implied in Judges 1:16.
Judges 1:19: And the LORD was with Judah; and he drave out [the inhabitants of] the mountain; but could not drive out the inhabitants of the valley, because they had chariots of iron. [drave...: or, possessed the mountain]