Initially, you will be sent an email every time anyone in your group adds a post to the forum (assuming that you have a valid email address enabled in your profile). The email will contain the text of the post. This is called being subscribed to the forum. If you want to unsubscribe (which means you won't get the emails, but you can still read and post to the forum), the quickest way to unsubscribe is to click the link on the email Unsubscribe from this forum.
Alternatively, you can choose to receive just one email per day, containing either all the day's posts to all the forums to which you are subscribed, or just the headings for the posts. This is called a digest, and allows you to scan through the posts quickly to see if there is anything to which you want to respond, without filling up your inbox too much. To choose this method, go into your profile, select Edit profile and press Show Advanced. Then choose one of the options from Email digest type.