Answer Description
A good password can be defined in many different ways - however at a minimum it should contain letters, numbers, special characters and be at least 8 characters long.
Wikipedia
Password strength is a measure of the effectiveness of a password against guessing or brute-force attacks. In its usual form, it estimates how many trials an attacker who does not have direct access to the password would need, on average, to guess it correctly. The strength of a password is a function of length, complexity, and unpredictability.Using strong passwords lowers overall risk of a security breach, but strong passwords do not replace the need …
Password strength - Wikipedia, the free encyclopedia