Not quite as cool as it sounds... It's a take-off on meta-cognition, and simply means thinking about memory. It is often specifically used to refer to knowing what you remember, or don't remember. This is a good thing, and it will improve your life. But really, it shouldn't be called metamemory.
I know that I don't remember what I ate for dinner yesterday (pitiful, I know), and that I do remember what I ate last thanksgiving. I know that I think I know what I ate for dinner the Christmas before last, but I also know that I am prone to high levels of repisodic memory. I know that I won't remember what I ate for lunch today the day after tomorrow.
And that's metamemory for you, folks!
If you want to work on improving your memory, you should find out about Mnemonics, the keyword method, Loci, and your basic memory; LTM, STM, and the sensory memory.