When an announcement is prepared and activated, those who are affected will see it at some point while browsing the site.
Use the "OK I got it" link to clear the screen. This marks it as having been viewed.
As soon as the announcement page interrupts a URL and is shown, it is marked as having being seen. There is only a 1 in 20 chance that announcements are shown on any page load, so not everyone will see them instantly.
This feature will be used sparingly so that it does not interrupt site users except on rare occasions.