Deuteronomy 20:11
And it shall be, if it make thee answer of peace, and open unto thee, then it shall be, that all the people that is found therein shall be tributaries unto thee, and they shall serve thee. And it shall be, if it make thee answer of peace, and open unto thee, then it shall be, that all the people that is found therein shall be tributaries unto thee, and they shall serve thee.
The instruction was part of the laws Moses recalled for Israel after forty years of wilderness wandering, just before they crossed the Jordan into the promised land. It stands within Deuteronomy 3 to 22, where Moses recounts the experiences of Israel and the guidance of God as one constant appeal to serve the Lord. That appeal calls Israel to turn away from the sins that wrecked the other nations and to exemplify Him, in whom alone lie greatness and continuance. Within that setting it belongs to the development of the precepts of the law joined with God's gracious promises. As such it reminds Israel of the sacred importance of God's law and the need to keep it.
In its plain sense, when a city makes answer of peace to Israel and opens its gates to them, all the people found therein shall become tributaries to Israel and shall serve them.