To visit any of the websites on Top Sites, click on the page thumbnail.
Top Sites was released with great fanfare a couple of years ago, it didn't really take off so you may not even know it exists. Some people really love Top Site, but to me, it is kind of gimmicky and of limited use. If you regularly follow a few websites, Top Sites might be for you.
You access Top Sites from the small 4 x 3 grid icon on the left side of the Bookmarks Bar. If you don't see this icon, you can add it to the Bookmarks Bar.
You can alter which pages show up in Top sites by clicking on the Edit button on the bottom left of the Top Sites page. You will then see an X and a push pin next to every thumbnail. Clicking the X will keep that page from ever showing in Top Sites. Clicking the push pin will keep that page always in Top Sites.