A strong password is a long password, of at least 13 characters. Numbers, capital letters and symbols make your password more difficult to crack. There are computer programs that try all possible combinations of letters and numbers at a dizzying speed until they find the right combination. Such a program can guess short passwords in minutes or even seconds.
If you have difficulty remembering a long password, you can create a passphrase. Choose a sentence that only has meaning for you and also contains unusual words, numbers and special characters.
If you use a password manager, you don't even have to remember your passwords at all.
Even if you use a strong password, your account is not always secure. How come?