Mid and low-fi are kind of ways to describe hifi. If something is low-fi, it sounds like crap and is not of any real worth. Think my-first-cassette-recorder sort of junk.

Mid-fi is more related to affordable yet still very good hi-fi. Generally, anything that is more expensive than what the layperson would buy but cheaper than what a massive audiophile would spend. For headphones, I would say anything from about £75-£300 would be mid-fi. That isn't to say that there aren't good products cheaper than that, they just don't fall under the mid-fi moniker.