A couplet is a pair of lines in a poem which have both the same rhythm (meter) and that rhyme. The lines can be independent sentences (closed form) or can run on from each other (open form).
This example from Rachel Rooney's Post shows the open form and consists of 2 rhyming lines with 10 beats each.
A queen in a palace, slumped on a throne,
Surrounded by servants but all alone.