Mental models is a persons model for understanding the world. Fx how long does it take the earth to pass around the sun? Now you are accessing your mental model of how the earth is in a path around the sun and thinking about how long time that might take.
Students try to make mental models when learning and sometimes have the wrong mental models. As teachers its important to understand the students mental models and correct them if they are wrong.
The mental models are made up of schema. Which is smaller templates of how the worlds looks and works. Fx a schema for dog includes how it looks, behaves, sounds etc.