It indicates that you have violated the terms of service. It might be something these days, from a statement you made to a photo you shared to the fact that you exploited the service in some way. If you’re aware that you’ve received an alert, the explanation for it will be in your help inbox. It would have appeared on the same message that informed you of the alarm. It follows you for as long as you have Facebook, and depending on what you did it for, you could be disqualified if you do it again. Only you can see the section of your profile with the banner that informs you that your account has a warning; no one else can see it. If your account has been limited in some way, the message will also inform you how long you have to wait. First-time offenders are given 24 hours, then three days, seven days, fourteen days, and finally thirty days. You could be given 30 days the first time, depending on the severity of your limitation.