Subscribing to a forum allows you to receive email notifications whenever someone posts to the forum. However, the ability to do so is dependent on the setting your instructor chooses for each forum. There are several settings that your instructor can choose from:
Tracking will show you on the course home page, on the main page of the forum, and within each thread, what posts you have not read yet. By default this is available, but your instructor does have the ability to turn it off for a forum.
Note: your instructor may have removed this block. If so, locate a forum in the course and click on the forum name. Click the Search Forums button at the top. At the very top of the page, in the breadcrumbs above the course name, click Forums.