A couplet or distich is a poem or a stanza within a poem which contains two lines of matched verse in succession. They can be matched in length, in rhyme, or both. The shortest couplet that forms a poem is perhaps "Lines on the Antiquity of Microbes" by Strickland Gillilan:

Adam
Had 'em.

Many couplet poems also have a title, which is key to the understanding of the poem as a whole. The two lines of a couplet typically complement or contradict each other in some way for dramatic effect, such as this poem called "Attempt"

At least try
To try

A couplet is also a pair of rhyming lines within a longer stanza.