Related: Fake explanation, Guessing the teachers password, Understanding your understanding, many more
The mental model concept gets used so frequently and seems so intuitively obvious that I debated whether to bother writing this. But beyond the basic value that comes from unpacking our intuitions, it turns out that the concept allows a pretty impressive integration and streamlining of a wide range of mental phenomena.
I think mental models are interesting for being mechanisms, rather than merely classes and instances. I suspect we may refer to the concept again, in which case a brief definition on the Wiki linking a straightforward elaboration such as this is useful.