Having purified your souls by your obedience to the truth for a sincere brotherly love, love one another earnestly from a pure heart, since you have been born again, not of perishable seed but of imperishable, through the living and abiding word of God; for
“All flesh is like grass
and all its glory like the flower of grass.
The grass withers,
and the flower falls,
but the word of the Lord remains forever.”
And this word is the good news that was preached to you.
What does it mean to be born again? Just before John 3:16 (For God so loved the world that He gave His one and only Son, so that whoever believes in Him shall not die but have eternal life), Jesus tells Nicodemus he must be born of water and the Spirit. Our human bodies are born of perishable seed just like the grass. The grass is beautiful and has a glory that fades, just as our bodies die. But you, being born again, of imperishable seed will live forever. So we put our hope in God's Word, not in earthly beauty.
Matthew Henry's Commentary:
"The apostle having given an account of the excellency of the renewed spiritual man as born again, not of corruptible but incorruptible seed, he now sets before us the vanity of the natural man, taking him with all his ornaments and advantages about him: For all flesh is as grass, and all the glory of man as the flower of grass; and nothing can make him a solid substantial being, but the being born again of the incorruptible seed, the word of God, which will transform him into a most excellent creature, whose glory will not fade like a flower, but shine like an angel; and this word is daily set before you in the preaching of the gospel."