A good password is difficult to crack or guess. A password must be at least six characters long. (Note that you can increase the required length and set other parameters for users' passwords on the Users Accounts--Password & Account Policies screen.) If you've chosen a good password, the longer it is, the more difficult it is to crack. Only the first eight characters are meaningful unless your system uses MD5 passwords. If MD5 passwords are enabled, your system will accept passwords that are longer than eight characters.
Good passwords contain a combination of letters, numbers, and special characters. It should use both upper case and lower case letters. Don't use your username, your anniversary, your social security number, your dog's name, your middle name or the word root. Don't use any variation of a word associated with your account. Don't use a word that can be found in a dictionary; dictionary words are easy to crack.
A simple technique for creating a password is to use the first letters from each word of a phrase that is familiar to you (a line from a favorite song might be appropriate). Insert a few numbers and/or special characters in place of letters and you'll have a decent password.