A "pwned" account is where a username and password you use with an online service has suffered a data breach. We're not talking about your RCNitroTalk account.
This isn't just the large organizations either, this happens to small, medium and large sites and services!
Once a breach happens the username/email address, and password should be treated as in the public domain.
If you are unfortunate enough to have been a victim of a breach you should stop using the password on that account immediately and change your credentials on any other services where you have used the exposed password.
We would recommend doing the following: