If you are seeing a message similar to "Your password was found in a data breach", it means our automated system has found the password online. When companies have a data leak, lists of usernames and passwords get leaked online by hackers. If a password is found on this list, our system will not allow the password because it is not considered secure.
If the password you are trying to use is one that you use in other locations, you should also consider changing the password on those sites. It is a good idea to use unique passwords for each site and to use passwords that are not easy to guess.
If you aren't sure about the security of an username or password, try visiting this handy online checker: https://haveibeenpwned.com/