what it means
Two and three level systems are more easily able to mimic the actual effects of removal of the various cards and thus are more accurate at estimating your actual advantage for betting purposes and has a better chance of predicting the quality of the next card as it relates to your hand for playing purposes. In statistical terms, for betting purposes a higher level system usually has a lower standard error in it's estimate of your advantage.
For example, a good two level system will win at anywhere from 5% to 15% faster than a basic one level system like High-Low. (depends on number of decks, penetration, and rules).