It appears on the front page of unread messages next to the page number links as well at the top and bottom of each thread page. But only for topics that have 100 or fewer messages.
As its name implies, clicking on it will display all of the messages on a single page.
It is important to keep the maximum messages at a reasonable number to avoid slowing forum response times. With 20 messages per page this means that "All" will be available for threads with 5 or fewer pages.
