Your password is the "key" to your phpList installation. Just as you wouldn't leave your house keys outside the front door, or your PIN number written onto your bank cards, security begins with effective password management.
A strong password is one that is difficult to detect by humans or computers. It is generally accepted that a strong password:
Is unique to this application (that is, don't use the same password that you use elsewhere).
Is at least eight characters long.
Contains numbers and letters in both upper and lower case. Consider using symbols too.
There are several websites that will help generate / validate strong passwords. (Just search the web for "how strong is my password?").