Fraud alerts are just what they sound like. It is an alert associated with your credit file. You will be alerted when your credit is accessed. It will normally last 90 days but can be extended if needed. If you are a victim of identity theft or at risk for identity theft, this should be one of your first steps.
After putting on the fraud alert, the next step is to set up a monitoring service. This is necessary because while fraud alerts are a good protection, unfortunately not all lenders will see it and act on it. They sometimes do not look at anything besides your credit score and will miss the fraud alert. A monitoring service will send you an alert whenever there is a request to your credit report. That way if it is somebody fraudulently accessing your identity you can stop it before they get too far.