What does Deuteronomy 3:18 mean?

At that time, I told you: 'The Lord your God has given you this land to live in. But all your fighting men must cross over ahead of your fellow Israelites, ready for battle.' Deuteronomy 3:18 - Modern Text Bible

(Moses reminds them that even though some tribes got their land early, their warriors still had to help the rest of Israel conquer the land west of the Jordan.)