What it is
A strong password should be made out of numbers and letters. Let's say Example logged on here with a password like 123. And someone cracked into his username. And that person vandalized Wikipedia (we wish that wouldn't happen) and he was blocked infinitively. Well, Example couldn't come back on here because a vandal got him blocked by vandalizing Wikipedia! A password should be made of numbers and letters. Let's say an administrator decided to unblock him. They decided that he's been a good user, and they unblock him. Example decided to change his password to something like 3fr69bz0hl. This way, he could keep this from never happening again.
What it is not
Example come on here one day and decided that he could change his password back to 123. That night a vandal got on here and vandalized Wikipedia with his username. Unless you want this to happen to you, give yourself a strong password! Otherwise, they may be dire consequences. You may be blocked forever.
What to do
If this ever does happen to you, find out who did this, if possible, and report to an administrator.