moodle/lang/en/help/forum/subscription2.html

<p align="center"><b>Forum Subscription</b></p>

<p>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 <?php echo $CFG->maxeditingtime/60 ?> minutes 
after the post was first written).
</p>

<p>People can usually choose whether or not they want to be 
subscribed to each forum.
</p>

<p>However, if you choose to force subscription on a particular forum 
then all course users will be subscribed automatically, even those that
enrol at a later time.
</p>

<p>This is especially useful in the News forum and in forums
towards the beginning of the course (before everyone has 
worked out that they can subscribe to these emails themselves).
</p>

<p>If you choose the option "Yes, initially" then all current and future
course users will be subscribed initially but
they can unsubscribe themselves at any time. If you choose "Yes, forever"
then they will not be able to unsubscribe themselves.
</p>

<p>Note how the "Yes, initially" option behaves when you update an exisitng
forum: Changing from "Yes, initially" to "No" will not unsubscribe
existing users, it will only affect future course users. Similarly changing
later to "Yes, initially" will not subscribe existing course users
but only those enrolling later. 
</p>