Keeping track of changes to Wikipedia pages would be prohibitive if you had to visit each one. That is what your watchlist is for. At the top of any page, you'll find a link to your watchlist, a reverse-chronological list of edits to articles and other pages you've edited or are interested in.
There is more than one way to populate your watchlist. While reading an article, click ☆ in the top tabs, and it will be added. Another method is to click "Watch this page" in the edit box of any page you're working on.
By default, the watchlist shows the most recent edit to your watchlisted pages, including the editor username or IP address responsible for it, plus date and time, and other information. If you want to see more than one recent edit, you can customize this in Preferences.
Most interesting, the watchlist will show links to specific edits ("diffs") to a page's history ("hist"). We'll talk more about these in coming weeks.