You have the option of receiving one email per post that is made in any forum to which you are subscribed, or receiving one email per day about forum activity ("Digest"). You can change this option by editing your profile settings.
To do this, go to the "Administration" tab on the left (tilted vertically). Hover your mouse cursor over Administration, and go to "My profile settings" --> "Edit profile". If you do not see the Administration menu, click on your name in the upper-right corner of the screen, and then you will see the Administration menu on the left.
From here, you can change the settings for "Email digest type". To receive one email per day, select one of the Digest options. To receive one email per forum post, select No Digest.