a number, math, nada
It is a precisely calculated number that is intended to allow those folks with boat that have lower values to sell there boats as "less likely to capsize than those other boats". This is supposed to make you feel all warm and fuzzy about buying the boat. It is actually just a measure of the beam to the ballast ratio which is intended to give you the ability of the boat to recover from a capsize. Most think it is the boats ability to resist capsize but it is not. It does not account for the hull to deck construction which is critical to not "going all the way over" in breaking waves It does not account for de-masting which typically allows the boat to right itself even if it capsizes It does not account for a lot of things. The general math is that wider boats of the same ballast will capsize more catastrophically than a narrower one or that boats of the same beam will be less likely to capsize catastrophically if they have more ballast. The extreme example is a catamaran. Once it goes over it stays capsized. It does not address that it is pretty hard to get one to go over that far in the first place. Lower values are "better" and anything over 2.0 is generally considered a "non blue water boat" for some reason not supported by the facts. I would point out that catamarans have values of 3-4 and go out on the open ocean just fine.
