Manual:Page change notification
With the watchlist table, for each watched page of each user the system keeps track of when it was last watched, in order to indicate whether changes were made which the user has not yet seen.
On the watchlist a page name is bolded if the page has changed after the last time the user concerned has viewed the page (while logged in).
If one views the history of a watched page directly, without first viewing the page, the edit at the top (the most recent one) may be marked with update marker "updated (since my last visit)" (or the content of MediaWiki:Updatedmarker). This applies if the edit was made by someone else and you have not viewed the page (while logged in) since it was made. The positioning of the message, suggesting that it is a property of an edit rather than a property of the page, is somewhat misleading, because not all edits which have not been viewed yet are marked.