The poem
Cherry-ripe, ripe, ripe, I cry, Full and fair ones; come, and buy: If so be you ask me where They do grow? I answer, there Where my Julia's lips do smile;-- There's the land, or cherry-isle; Whose plantations fully show All the year where cherries grow.
he compares his lover's lips to the lusciousness of the ripe cherries
Robert Herrick compares his mistress`s lips to rip cherries. by comparing his mistress to a cherry Herrick emphasizes her freshness and beauty which are always the same even in the course of years.