“Mark, Lakshmaṇ, mark how Sítá’s breast With eager longing is possessed. To-day this deer of wondrous breed Must for his passing beauty bleed, Brighter than e’er in Nandan strayed, Or Chaitraratha’s heavenly shade.

In plain words: Rama tells Lakshman to see how badly Sita wants the deer. He says that today it must be killed for its beauty, since it is lovelier than anything in the gardens of the gods, like Nandan.