To me, the term "Middle English" refers to a stage in the evolution of the English language. Middle English is the language of Chaucer (1340-1400) and Malory (1415-1471). Chaucer wrote "The Canterbury Tales." Malory wrote "Le Morte d'Arthur," the book about King Arthur, the Knights of the Round Table, and the quest for the Holy Grail. It is really impossible for me, or for most modern speakers, to understand Middle English. If we see a translation into modern English, then we can see that, yes, it really is English. For example, the first lines of the Canterbury Tales are: Whan that Aprill with his shoures soote The droghte of March hath perced to the roote. With a little help, we can say "Oh, 'whan' is 'when,' 'shoures' is 'showers, 'droghte' is 'drought,' 'hath' is an outdated form of 'has,' 'perced' is 'pierced,' and 'roote' is root." We probably couldn't guess that 'soote' is 'soft.' And when we put it all together-- When that April with its showers soft, The drought of March has pierced to the root, --well, it's still difficult. And the next sentences are worse. If we just update the spelling of the words, we get something like "And bathed every vein in such liquor Of which virtue engendered is the flower." For me, and for most native English speakers, the limit is about 1600. We are expected to understand the English of Shakespeare (around 1600) and the famous 1611 King James translation of the Bible. But it's difficult, and we need help. There are old verb forms like "thou" and "thee" and "hads't" that we don't use any more, for example. The language is always changing. In the 1930s, literary English became much simpler, for example. Understanding English from the 1800s can be difficult. Understanding English from the 1600s is a challenge. Understanding English from the 1400s? No, it's really impossible to understand, it's just too different.

Dan Smith has given you an excellent answer. "Middle English" is essentially English after the Norman Conquest (i.e. massive French influence) but before the establishment of modern spelling, grammar, and pronunciation. The version of "Le Morte d'Arthur" published during the late Middle English period is still readable today, although publishers usually update the spelling, and actual conversation between King Arthur and a present-day native English speaker would be extremely difficult, due to pronunciation and slang in particular. Also the technological / political / social / economic context would be rather confusing.

Phil and Dan gave excellent answers. Middle English is basically a foreign language to modern English speakers.