My screen has a box on it saying Check Signal Cable. What does it mean?

Check that the blue plug-in connectors are tight and in right. double check all connections are tight too. Next if that doesn't work if you can reseat your memory inside the tower and or your video card. If you don't know how to do that youtube has everything...search for "reseating video cards in a PC". People on here gave me the same advice and it worked for me.