Metacognition is thinking about thinking.
For example, if you're wondering why math homework is harder than geography, you're already thinking about your own learning process: you're thinking about thinking.
John Flavell wrote in 1976 that metacognition is based on both metacognitive knowledge (about yourself, tasks and strategies) and metacognitive experiences. That is hardly practical. After all, knowledge is knowledge: if you have the knowledge about your thought process, it is still only knowledge. So once you get past the big "aha" moment, there's nothing special to see. Recursivity is absolutely cool, but thinking about thinking isn't magic. Flavell also admits that only the context determines whether cognition is "meta-cognition", and that really isn't of much importance. For example: your experiences with various learning strategies is knowledge, and as such not so different from your experiences with geography.

Recursivity, Descartes and ...

I remember the first time in programming when I had that "aha" moment for recursiveness: it was about a piece of code, using itself to arrive at a solution.

For non-programmers, a good example of the use of recursion is sorting pages alphabetically. First, you put together those pages that have the same first letter, and then you repeat this process for each stack, until the substacks are manageable. Would you describe that process, it could look something like this:
1. When the stack is sorted, you're done with it.
2. Walk through the stack, making sub-stacks based on the first letters.
3. For all new sub-stacks, use this same process, adding an extra letter.

There are also recursive acronyms, eg "Visa" on your credit card stands for "Visa International Service Association". There's also Dilbert's famous TTP, which stands for The TTP Project.

So if, in the world of learning, metacognition is just thinking about thinking, then metacognition is also about metacognition itself, because what else would you call it?
If you want to read more about recursivity, read the "recursivity" section again. Just kidding.

A discourse on Descartes

Descartes' statement "Cogito, Ergo Sum", the "I think, therefore I am" came to him as an Aha! In his time it was fashionable to question everything, including thinking. But in order to doubt it had to exist: therefore you cannot doubt existence itself. Together with Descartes, we can conclude the same, and transfer that insight to metacognition: not only do we know, we also know that we know.