A couplet is two lines of poetry that usually rhyme. Here's a famous couplet: "Good night! Good night! Parting is such sweet sorrow / That I shall say good night till it be morrow."
The couplet above comes from Shakespeare's Romeo and Juliet, which is a play, not a poem. But Shakespeare often used rhyming couplets at the end of scenes to signal the ending. Couplets are very common in poetry. Often whole poems are written in couplet form — two lines of rhyming poetry, followed by two more lines with a different rhyme, and so on. Robert Frost, one of America's great poets, wrote many poems using couplets.