What is a couplet in a poem mean?

A couplet is generally used in poetry.  As indicated in
the term so similar to "couple," it refers to two lines paired together, and in poetry
these lines rhyme.

For example, in Shakespearean (aka
Elizabethan) sonnets, the last two lines of this 14-line poem rhyme.  The author will
many times use these two lines at the end of the sonnet to summarize the ideas presented
in the first 12 lines, so that the rhyming couplet may act much the way a conclusion
would in an essay.

The rhyming of the couplet occurs at the
end of the first line, coinciding with the sound at the end of the following line
(called "end rhyme").  It can be a perfect rhyme (such as "car" and "far") or a near
rhyme (such as "car" and "fair").

For an example, search
William Shakespeare's Sonnet #29, which begins, "When in disgrace with fortune and men's
eyes..."  The last two lines are examples of a rhyming couplet.  After reading this
"poem," you hear the pleasant chord of the rhyme, but you will also see the power with
which Shakespeare drives his final point home.  It's really beautiful, and surprisingly
so, made up of only two lines that rhyme!