Couplet is a fancy word for two lines of poetry of equal length, one after the other. Sometimes it's necessary for rhyming, sometimes it's not necessary for rhyming.
Example:
- Oh how the sun come shining above/Showering me with a sky full of dove.
- People say madness is not the cure/And I think either is being pure.
- I hate homework so much, it drives me crazy/But I think not doing them, is called being lazy.
Couplet is the most common thing people would learn in Language Arts, it helps people get familiar with poetry and rhyming.