Turning a thread into an announcement displays it on every board on your forum, provided that board has announcements enabled.
When creating a thread, you can automatically make it announcement by selecting the icon on the Create Thread page and selecting Make Announcement. You can have the announcement last until you remove it or you can give the announcement an expiration date by checking the box and adding a date in the window that pops up. Once posted, the thread will already be an announcement.
To make an existing thread an announcement while you're viewing it, click on the Actions dropdown at the top of the thread. Select Moderate, and then select Make Announcement.
To make multiple threads change into announcements, visit one of your boards and select the checkbox next to each thread you want to make an announcement. Click on the Actions dropdown, select Moderate, and then select Make Announcement. This will affect every thread you selected.
These same steps can be followed to change an announcement back to a regular thread. The menu option to make the announcement will simply be changed to Remove Announcement.