Over 700 finance terms, Shmooped to perfection.
It's an index. Just an index. It doesn't mean that much other than that it is an index widely quoted in the financial press. Specifically, it is the number of stocks which are up vs. the number which are down (hence the catchy "advance" and "decline" nomenclature). You can imagine though how this index has wide ranges for error - i.e. every stock could be down one penny and the ratio would be 0 which would be like "as bad as it's ever been" - when in fact the day was basically Olive Oyl flat.