Password strength is scored on varoius factors. Through pattern matching and conservative estimation, it recognizes and weighs 30k common passwords, common names and surnames according to US census data, popular English words from Wikipedia and US television and movies, and other common patterns like dates, repeats (aaa), sequences (abcd), and keyboard patterns (qwertyuiop).
However no one of these elements alone will necessarily make a pasword weak. The overall strength is indicated by the color of the bar. As long as the bar is not red, the password is acceptable as long as it meets the other requirements.