The Register command brings up a dialog box that lets you personalize your copy of Sloop. This dialog box contains fields for your name and registration number. To obtain a registration number, you must pay for Sloop. For more information about paying for Sloop, see the topic Why and how to Register.
Once you register Sloop, the register command is no longer available and your name will appear in Sloop's about box. In addition, an annoying shareware notice that appears every once in a while will no longer appear.