What does it mean couplet in romeo and Juliet?

Actually, the Word "couplet" does not appear in Romeo and Juliet, and on the only two occasions where Shakespeare uses it, (Hamlet V, 1 and Twelfth Night III, 4) its meaning is far from clear. It means a pair of something, in the Hamlet quotation eggs and in the Twelfth Night one lines of a song. Maybe. However, even if Shakespeare did not use the Word, we use it to describe a pair of lines which rhyme such as "The grey-eyed morn smiles on the frowning night,/Chequering the eastern clouds with streaks of light.", the first two lines of Friar Lawrence's monologue at the start of II, 3 which is written entirely in couplets.