This message indicates that there may be a problem with the certificate issued to the server to which you are connecting. The certificate may not be trusted; it may have expired or been revoked. If this is the case, you must resolve these problems before you can connect to the server.
This message can also appear if you are connecting to the server by using a name that does not appear on the server's certificate. If this occurs, it can seem that you are being maliciously redirected to a server other than the one to which you had intended to connect. You are given the option to connect, but you should view the certificate to determine whether to connect to the server.
This message can also appear because of some combination of the errors listed above. If this happens, use the View details link to investigate the details of the problem and determine the best course of action.