A sestina is a poem with a very rigid structure of 39 lines, divided into 6 stanzas of 6 lines, with one tercet (3-line stanza) at the end of the poem.
Throughout the poem, 6 particular words are repeated at the end of each line. That is, in the first stanza, there are 6 different words, one ending each line. One of these words ends each line in each subsequent stanza, but the endings are not repeated within the stanza.
This repetition has a pattern. In a true sestina, if the words at the end of the lines of the first stanza are labelled 123456, the words at the end of the lines of the second stanza would be 615243, followed by a stanza of 364125, then 532614, then 451362, then 246531.
These words are then repeated in the final tercet, usually two to a line. Thus, the words chosen to be repeated so often are generally very pivotal words to the poem.