What are Rhyming Couplets?

A pair of lines is called a couplet in poetry.
A rhyming couplet is two lines of a poem that rhyme.
Rhyming words that are similar sounding words are used at the end of the two lines to make it a rhyming couplet.
For example: The sky is blue, the valleys are green,
                     There is such beauty nowhere else to be seen!