This means that you only need to remember that single password to gain access to all the rest, allowing you to set up strong and different passwords for each of the systems, sites and services that you use.
One such password keeper is called LastPass, a free app that stores your 'password safe' online on the LastPass server, protected by a Master Password that you use to open this online safe.
Other password keeper apps, such as 1Password and OneSafe store your password safe on your device, with a Master Password that is only known to your device and is not stored online.
I prefer the second style of password keeper, because I have always been nervous about storing my valuable set of passwords online - in case of attack by hackers targeting the site that holds my password safe.
Well, it appears that such a breach by hackers has occurred this week with LastPass, where Master Passwords (as well as email addresses and other information) were 'compromised'.
LastPass is notifying users of the service if they need to reset their Master Password, and to also change their password on any other online account that uses the same password.
Here is the full article from MacWorld this week.