Yes! Offline mode is a must-have feature for when you need to do maintenance work, or if you want to register your domain name before you've finished creating your website.
Go to the Editor and click on Settings.
Under General you'll see the option Offline mode, which is disabled by default. Enable this by clicking on the slider to take your website offline.
Once you've enabled Offline mode, you'll see some additional options. The Message field allows you to decide what your visitors see if they access your website while it's offline. You can also opt to password protect your website by enabling the Password access option.
Don't forget to save your changes when you're finished! If you want to put your website back online, follow steps 1 and 2 again to disable Offline mode.