What does Genesis 20:13 mean?

Back when God led me away from my family and my old life, I told Sarah, 'Wherever we go, let's agree to say you're my sister.' That was the deal we made to protect ourselves. Genesis 20:13 - Modern Text Bible

(Abraham is explaining to Abimelech why he and Sarah claimed to be siblings. He frames it as a survival strategy they agreed on when God called him to leave his homeland.)