Recall (see 1.4.1) that the Back button makes your browser go to the page which you visited just before the current page. The browser remembers where you were before you pressed Back, and Forward just takes you "forward" there again.
Why is Forward usually greyed-out (i.e. disabled)? Because usually you are viewing a page as a result of following a link to it, and your browser considers this to mean that the current page becomes the "most recently seen page of all". In other words, following a link makes your browser forget any Back steps you just did. So "undoing" the most recent step Back just doesn't make sense.
Experiment with these buttons a little bit now to get the hang of it.