A refrain in a poem is a specific word, a group of words, a line, or a group of lines that is repeated in certain moments within the poem. In writing songs, it’s usually used to make it catchier and easier to remember.
The refrain is usually placed at the end of the stanza or serves as one stanza in between the other stanzas. It can also include rhymes. An example of a poem that uses lots of refrains is One Art by Elizabeth Bishop.