The metaself is the self that judges, compares and measures the self against its environment and its ideas. If this sounds like a rather nebulous and unspecific notion, that is because it is. The metaself tells the self everything from the fact that it would be a much happier better self if it had a nice big plato of nachos right now to the fact the self is still abstractly not fully embracing the environment around it in a moral sense.
As a notion, the metaself is pretty useful, because, really, there isn't much self that isn't meta. The only self that reacts to its environment without first comparing and contrasting what type of self it could possibly be is the self that pulls its hand away from hot stove. Everything above the level of reflex is reflection.
The main interest in understanding the metaself is how it is acknowledged or denied in various societies, either on a theoretical level or in day to day life.
For example, in American society, most people would draw a distinction between selfish acts, acts performed solely for the benefit of the self, and charitable acts, those performed for the benefit of others. However, most acts that are considered "selfish" are more analytically "metaselfish". For example, someone who wants an expensive car is behaving in a metaselfish way, because their metaself believes that the most desirable self to have is the one who has a fancy car. It could believe this for several reasons, including how others (capitalize that if you like) perceive the self, or perhaps based on an ranking scheme it has designed in which this particular self should have a certain status symbol. A person who wants power and a person who feeds the poor are both doing what their metaself thinks should be done with the self, only in some cases the metaself is working with more or less confusion.
Another interesting point to note is that in my country, the United States, it is a hallmark of the intellectual elite that they are aware of, and know how to use the metaself to a great degree, almost to the point where they forget that they have an actual self. They train themselves, and pride themselves on their ability to remove themselves from their circumstances and view things with impartiality. Of course, a byproduct of this (and someone should perhaps research the historical progression of this) is that they view, either covertly or overtly, anyone who doesn't have this attitude as a barbarian, whether it be those gangsta rappas or those Montana Rednecks, depending on the specific politics of hypermetaselfized American intellectual. However, as I said above, the metaself is actually a natural part of the self, and everyone, even those who don't know it (and wouldn't know what the word "meta" means) has a metaself. The macho men who are the great fear of the modern intellectual are even more overtaken with having a metaself that measures and categorizes their progress, just in a different, and often detrimental way.
On the note of gangsta rap, it was a sometimes gangsta rapper, the infamous KRS-One who taught me that the role of self and metaself are interchangable, and that the desire to embrace all forms of life and understand them is the same desire to preserve your life by killing someone.