Myo,
When a person is subscribed to a forum it means that they will be sent email copies of every post in that forum (posts are sent about 15 minutes after the post was first written).
People can usually choose whether or not they want to be subscribed to each forum.
However, if the editing teacher chooses to force subscription on a particular forum then all course users will be subscribed automatically, even those that enrol at a later time.
In their profiles, people can choose to disable their e-mail and then they will not receive e-mails.
If this does not solve your problem, look here:
http://moodle.org/mod/forum/discuss.php?d=37536
Hope this helps! Moodle on!
--Mary