This term seems to originate with electrical engineering and I won’t try to understand/define that usage at the moment. When it comes to software development it seems to indicate a difference in the types of data being communicated by two disparate systems (e.g. a programming language and a relational database; oftentimes with one offering a limit subset of the abilities offered by the other). See this article for an example usage of the term impedance.
One could say instead of “impedance mismatch” that the two systems have differing data representations resulting in difficulty implementing translation from one system into the other.