A strong password is a password that resists easy access by trial and guesswork. Many IT systems have requirements for strong passwords, where users may have to input more complicated passwords in order to prevent unauthorized access. Strong passwords can help to protect not only individual user accounts, but comprehensive hosted systems.
Techopedia explains Strong Password
There are many components in a strong password strategy. The most basic ones involve length and diversity. A longer password is inherently stronger because it contains more characters and there are more permutations involved. Adding a requirement for uppercase and lowercase letters, numbers or special symbols also makes passwords stronger. For a basic example, a generic four character password like "1234" could be easily guessed, and fraudulent users or hackers could use that to invade a system. A stronger password might involve the above approaches for a result like "1two34!" which is going to be much harder to guess.
Although strong passwords do help to protect systems, newer approaches are gaining popularity. One major example is biometrics. By adding biometric input, systems administrators can be more confident that they are only allowing the legitimate parties to access the system. Some IT administrators have also started adding more like channel approaches like out of band authentication, where various parts of passwords may be sent to different devices, for example, a laptop computer and a cell phone through different telecommunications networks.