Kenneth Craik talks about mental models in The Nature of Explanation, where he introduced the concept in 1943 (http://uxmag.com/articles/the-secret-to-designing-an-intuitive-user-experience).

The mental model describes how the user understands a system or how a thing works. Mental models are built upon experiences and interactions that the user has had in the past. It gives the user a personal perspective and a way to understand what is happening at each moment. This personal point of view gives the user an idea about how to interact with the system and how to solve future situations.

In some cases, your future users will not have any previous experience with your system, but even in those cases, the users will have a personal view about how it should work. This understanding emerges from experiences with similar systems, and generates user expectations. Users will feel lost if the system does not act as they expect.

You can close the gap between the user mental model and how the product is designed by explaining how the product works to them before they get lost during their first use. Welcome processes and just in time tips can help the user learn how the product works as they interact with the system, adjusting their mental model as they experience their first steps with your product.

Skeuomorphism has been used in mobile app design for years to reduce the gap between the new devices that were emerging and the previous user experiences. This technique consists of introducing elements of real-life objects in the product user interface, so the user recognizes them even before beginning to use the application (https://en.wikipedia.org/wiki/Skeuomorph).

It has its advantages and disadvantages. While it may assist the user in recognizing the operation of certain controls by past learned behaviors, it may also be that these controls are more difficult to handle as they pass from the real world to be represented on a touchscreen. We have to evaluate whether it is worth using this technique, or if we show other controls more adapted to the platform for which we are designing. Also, keep in mind that touchscreens have been around for a long time, so the operation of many controls is already familiar to our future users.

For example, if we are designing a music player app, we can use the levers and buttons that we would find on a real radio. When the users find radio controls, although these controls are displayed on a screen, they already know how to interact with the system. Once the user becomes familiar with the location of the elements and their functions, we can substitute the physical aspect of the elements for others that are more minimalist, obtaining gradual learning.

There is a well-known example of this kind of evolution in Apple's operating system. When Apple launched the release of iOS 7, many of the elements that gave a realistic touch to the user interface were removed, but users already knew how to interact with a mobile after years of experience. Touchscreens were already something common for most users, and therefore not a strange device anymore.