An epigraph is a brief bit of text, usually borrowed from another writer, found before a poem, but after the title. (You may also find one at the start of a book, before the poems, but after the title page.) It gives a reader, or listener, something else to hold in mind as the poem is read. Neither part of the poem, nor wholly separate from it, an epigraph can be used for various purposes; it can be necessary information to understand a poem, for example, or it can be something with which the poem disagrees.
It is predominantly found in written form, but in a reading, the poet may expand upon the epigraph, as Adrian Mitchell does in 'Life is a Walk Across A Field'; the proverb with which the title disagrees is an epigraph in the published poem, but here Mitchell explains that instead.