Your mental models are your theories about how the world works. They come from your life’s experiences, your education, your family, your cultural background, your work experience, your religion (or no religion if you don’t have one). For most people, your mental models are sub-conscious – they affect how you behave but you aren’t aware of the impact that they have.
For example, if you had a mental model that as a manager you should have all the appropriate knowledge of someone in that role to justify your title and the money you are earning, and a staff member asks you a question to which you do not know the answer, then you are at risk of responding with a lie. You will simply make up an answer that will re-enforce your view of yourself as a competent manager.
This short video explains this concept in more detail.