What does Deuteronomy 4:43 mean?

You shall keep his statutes, and his commandments, which I command you today, that it may go well with you, and with your children after you, and that you may prolong your days in the land, which Yahweh your God gives you, forever.

Then Moses set apart three cities beyond the Jordan toward the sunrise;

that the man slayer might flee there, who kills his neighbor unawares, and didn’t hate him in time past; and that fleeing to one of these cities he might live:

This is the law which Moses set before the children of Israel.

These are the testimonies, and the statutes, and the ordinances, which Moses spoke to the children of Israel, when they came out of Egypt,

beyond the Jordan, in the valley over against Beth Peor, in the land of Sihon king of the Amorites, who lived at Heshbon, whom Moses and the children of Israel struck, when they came out of Egypt.