Then spake the king of brooks and seas To Raghu’s son in words like these: “Now let a wondrous task be done By Nala, Viśvakarmá’s son, Who, born of one of Vánar race, Inherits by his father’s grace A share of his celestial art.

In plain words: The god of the Ocean speaks to Rama, the son of the Raghu family. He says that Nala, a monkey who is the son of Vishvakarma, the builder of the gods, has inherited some of his father's heavenly building skill.