A Disk is an Item in Lunacraft dropped by Astronauts when killed. You also start out with one. They give tips, but the wiki is, of course, better.
When collecting a Disk, be sure to place it in the Scanner in order to get information from it. If you hold it in your hand and attempt to place it, use it, etc., it will give you a message saying,"Scan Disk to Decrypt Data."