This is a .net port of the zxcvbn.js library.
This lets you use the same password validation, on the client side, as well as server side.
The zxcvbn password checker is not based on how many leters, numbers etc you have in a password but on the real world entropy. In other words it takes into account dictionary attacks, common substitutions (l33t speek), keyboard patterns etc.
Entropy is measured in bits, and relates to the number of brute force atempts a hacker would need to try to find your password.
See here for the orignal motivation (and a much better explanation), behind the zxcvbn.js library.