How to understanding "Understanding the Yield Curve"

Hi, everyone. Have you read “Understanding the Yield Curve” by Salomon Brothers? I found the below part is difficult to understand. Shall we discuss it?

Quote: "The implied spot rates one year forward are, by construction, equal to such future spot rates that would make all government bonds earn the same return over the next year as the (riskless) one-year zero. "

Thank you very much!