To misname something is to give it a name that is wrong — either factually incorrect or, more often, misleading about what the thing really is. Note the emphasis: this is not usually about spelling someone's name wrong or calling your friend by their brother's name. It is about a name that does not fit the thing, so the name itself teaches people something false.

So you meet it most in writing where someone is correcting the record: a plant misnamed in an old catalogue; a law misnamed as a reform when it does the opposite; a disease that was misnamed for the place people wrongly thought it came from. Very often it appears as the past participle after be — "the street is misnamed", "the whole policy was misnamed from the start" — and in that shape it works as a small, dry piece of criticism: the name is doing propaganda, and the writer is pointing at it.

Be honest with yourself about how rare this word is. It is uncommon, formal, and slightly literary; in ordinary speech English speakers say "it's got the wrong name" or "that's the wrong name for it", and for the person-calling-you-the-wrong-thing situation they say called me by the wrong name. The related noun misnomer — a name that does not fit — is actually much more common than the verb, and if you are going to learn one of the two for your own use, learn that one.