In many cases such as testing new features, adding content, making a new design, etc. you may want to put your website offline or under maintenance in order to hide the building process from your visitors.
Joomla gives you an easy, one-click way to put your website offline. All you need is to log in Joomla Administrator panel -> Site -> Global Configurations and you will see a Site Offline option in there with two buttons Yes or No. Select the Yes button to make it offline and once done, the No button to put it back online.
Moreover, you can opt to put a custom message in HTML in the field there for your visitors to see.
