The rust colored banner above is an example of a notification. You may create any number of notifications and can either display them on all pages in your site OR add a notification block to one or more pages on your site to display them only on particular pages.
Where notifications show up is controlled by where you place Notifications blocks. When you place the block, you will be asked to choose a "Location". This refers choosing one of the "Display Locations" you have previously added to your site. If you do not choose a location, all live notifications on your site will be displayed in you new Notifications Block.
Once you have placed a notifications block on your site, you can go to the Notifications link near the top of the admin sidebar. This will take you to a list of all notifications for your site and allow you to add new notifications. A notification consists of the notification and one or more 'display locations'. You can set a notification to be displayed on all pages of the site by choosing the special "All Pages" display location when you create your notification. The code for placing a notification on all pages is included in the site's theme so you do not have to manually place a Notification block if you are using the "All Pages" display location.
If you want to start displaying your notification immediately, check the "Is Live" checkbox and save the notification.
Just as you can set pages to publish and unpublish themselves at specific times, you can set the "go live" and "expire" times in the "Publishing" tab when creating your notification.
NOTE: If you do not click the "Is Live" checkbox, your notification will not appear regardless of the publication date.