Metacognition is "thinking about thinking". It is being aware of what and how we think and why we think in a particular way. This is a skill that can compound very well if used properly. Metacognition is critical to learning. We need to engage different ways of learning for different type of knowledge we are trying to acquire. Reflecting on the very steps of learning and the acts involved is part of metacognition. Spending time on reflection after tasks that were particularly testing the limits of your ability is a good way to practice metacognition.
One key aspect of metacognition is self-regulation. This can be split into 3 parts; Planning, Monitoring and Evaluation. Planning involves trying to understand the problem and choosing the right set of strategies and tasks. Monitoring involves using your own awareness to judge during the process how effective you are. Evaluation refers to appraising the final goal compared to the outcome of the task and figuring out how it can be improved the next time around. This fits well into. The mental models are the set of tools you would have to use to solve a problem. Each time you perform a self-regulation you are in a way sharpening some of the tools bit by bit.