The noun sestet means the six final lines of a sonnet, or another group of six lines of poetry. You might discuss a sestet during a college literature class.
Use sestet to talk about very specific lines of verse, the last six in a sonnet. It's most common to find a sestet in Italian sonnets, such as those written by Petrarch and Dante. In English poetry, it's more usual to see a couplet — two lines of verse — at the end of a sonnet. The noun sestet occasionally fills in for the word sextet, or group of six things. The Latin root is sextus, or "sixth."