A pantoum poem consists of 4 or more stanzas.
Each stanza has an ABAB rhyme pattern.
The 2nd and 4th line of each stanza is re-used as the 1st and 3rd line of the next stanza.
The pattern goes on for as long as you like until the last stanza, where the 2nd line and 4th line are re-cycled from the first stanza. The first line of the poem becomes the last line, and the 3rd line of the poem is repeated as the 2nd line of the last stanza.