What does Joshua 17:15 mean?

Joshua answered, 'If there are so many of you, and the hill country isn’t enough, go clear out some forest in the land of the Perizzites and Rephaim. Make more space for yourselves.' Joshua 17:15 - Modern Text Bible

(Joshua tells them to take initiative: if they need more room, they should clear the forested areas nearby, even though those places are still occupied by other groups.)