The sense is: “Keep the covenant faithfully, and so shall your own and your children’s days be multiplied as long as the heaven covers the earth.” The promise of Canaan to Israel was thus a perpetual promise, but also a conditional one.
Related Commentaries of Deuteronomy 11:21
Deuteronomy 11:21
21 That your days may be multiplied, and the days of your children, in the land which the LORD sware unto your fathers to give them, as the days of heaven upon the earth.