A mismatch is two or more things put together that do not fit each other. The important part is that neither one has to be bad on its own. A very strong player and a very weak player make a mismatch; both are fine, the pairing is not. That is what separates this word from something like fault or error — a mismatch lives in the relationship between things, never inside one thing alone.

The pairing can be almost anything. Skills and a job: "a mismatch between the training and what the work actually needs". Two teams of very different strength, so the game is no contest. Colours or clothes that clash. Expectations and reality. And, very commonly in modern life, data: your name in one system and a slightly different spelling in another gives you a mismatch, and the computer refuses you.

The usual shape is a mismatch between A and B. That between is worth memorising, because it carries the meaning — it says the problem is in the gap, not in either side.

There is a verb too, though it is much rarer than the noun and shows up mostly as the participle: mismatched. The noun is what you will actually use.

Register is middling. It is a normal, useful word in news, work talk, and technology, a little more analytical than everyday speech — where you might just say "they don't go together" or "it doesn't match".