Couplets are two lines of poetry that rhyme. 
When you hear 'couplet' think 'couple'. For Example:

Son dos líneas de poesía que riman
por ejemplo...

One two
buckle my shoe

Three, four,
shut at the door

Caballito blanco, llévame de aquí.
Llévame a mi pueblo donde yo nací.

Hey diddle diddle,
the cat and the fiddle,

Bate bate
Chocolate

Jack and Jill
went up the hill

¿Qué te pasa
calabaza?
Nada nada
limonada

Humpty Dumpty sat on a wall,
Humpty Dumpty had a great fall;

You can put many couplets together for a longer poem:

All My Great Excuses
by Kenn Nesbitt

I started on my homework, but my pen ran out of ink…
My hamster ate my homework…My computer’s on the blink…

I tripped and dropped my homework in the soup my mom was cooking…
My brother flushed it down the toilet when I wasn’t looking…

My mother ran my homework through the washer and the dryer…
An airplane crashed into our house… My homework caught on fire…

...

I worked on these excuses so darned long my teacher said,
"I think you’ll find it’s easier to do the work instead."