MMA it means Mixed Martial Arts like Ultimate Fight Club
Pound for Pound means, judging purely PER POUND OF WEIGHT one fighter is better than all the rest. Read that again please. And again. This means that a specific fighter, regardless of what weight he actually fights at, would be the favorite to defeat any/every opponent at any weight. Now how you judge that 'per pound basis' concerns many different aspects such as, total wins, winning streaks, number of fights, class of opponent, actual weight, fighting styles etc.Lets put this into an example if you still don't get it. Imagine GSP at heavyweight where no other dynamics have changed except for his weight and size, which has risen in equal proportions. He is athletically and technically the same fighter, just in a different weight class. Would you consider him better than fedor, for example, the best in that division. Obviously the term pound for pound is very subjective but this is what the term is supposed to indicate; All things being propotionatly the same, which fighter would be the best across all weight classes.