A demo store is where there are products but the order done by customer is not processed. If your website is in development mode then it is better to enable demo store notice.
Enabling demo store notice in Magento is very simple. You just need to select an option in configuration settings in admin panel.
Go to System -> Configuration -> Design -> HTML Head -> Display Demo Store Notice
By default, it is set as ‘No’. To enable demo store notice, select ‘Yes’.
Now, you will see a notice bar at top of your page saying ‘This is a demo store. Any orders placed through this store will not be honored or fulfilled.‘
Hope it helps. Thanks.