Middle English is the stage of the English language used in poetry and other writing from roughly the late 12th century to the late 15th century. It is the language of Sir Gawain and the Green Knight and Chaucer, and it differs from modern English in spelling, grammar, pronunciation, and vocabulary.
For readers of poetry, knowing Middle English helps make older texts easier to hear and understand, especially when rhyme and meter depend on pronunciations that no longer sound the same today.
Example
Chaucer's The Canterbury Tales is written in Middle English, with lines such as "Whan that Aprill with his shoures soote," which looks and sounds very different from modern English.