A mental model is an explanation of someone's thought process about how something works in the real world. It is a representation of the surrounding world, the relationships between its various parts and a person's intuitive perception about his or her own acts and their consequences. Mental models can help shape behaviour and set an approach to solving problems (akin to a personal algorithm) and doing tasks.
Questions having the mental-model regards users' mental model and their representation of things and expected behavior.
Reference: Wikipedia - Mental Models