Your watchlist is a list of the pages you are watching to keep track of changes. By clicking on "Watchlist", you can get a summary of changes made to each of the pages you are watching. This way you can see if anything happened on pages you're interested in, without having to click through all of them one at a time.
Getting email notification of changes
In your preferences, you can set OMPWiki to email you when a page on your watchlist has been edited. It only emails you once until you view the page again while logged in, so you won't be inundated with emails if someone is making a bunch of changes.
Adding to your watchlist
When you are logged in, a "watch" tab appears at the top of every page you are viewing. Clicking it places that page on your watchlist, sends you to a message telling you that you've added a page to your watchlist, and changes the tab to "unwatch" the next time you view that page.
wikipedia:Help:Watching_pages for how watchlists work on Wikipedia.