People perceive the world through their senses. This constant stream of information is then processed by the brain to form a 'mental model'.
This is how a person makes sense of reality.
The power of a mental model is that it allows a person to predict the outcome when a certain event happens.
For instance, someone riding a bicycle 'knows' that if they lean slightly to the left the bike will turn to the left. If they grab the brake lever, the bike should (hopefully) slow down. They have built a mental model of how a bike works, they can imagine themselves leaning into a corner and the bike will behave as expected.
This mini-web takes you through how this mental model applies to human-computer interface design