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.
Another video about what mental models are and how students use them:
A video about how mental models affecting learning
Example of creating a mental model to help students learn
Check out some of the mental models in this article to get a sense of what a mental model is