Subscribers on all board types can be either board level or pulse level. Only team members can be subscribers on the board level. Guests can only be subscribers on the pulse level for boards that they have been invited to.
On the board level, subscribers are the team members who will be notified via the Inbox, the notifications bell, and email (if they are set for email notifications) whenever there are updates created within the board.
Being subscribed to a board also helps when separating your Main Boards view. You can see either "All" boards or only "Subscribed" boards.
To subscribe yourself or others to a board, you just need to click on the blue “Subscribe” button on the right of the board. If you are already subscribed to a board, this button will be white and say “Unsubscribe” instead.
When you create a board, you will automatically become the owner of a board, and you will need to assign another subscriber as an owner if you want to unsubscribe.
To see a list of all the board’s subscribers, just click on the icon to the left of the Hamburger menu.
On the pulse level, subscribers are the team members involved in a conversation inside a pulse. Those members will get notifications from the pulse to their email whenever someone updates the feed.
When you create an pulse, you will automatically become a subscriber to that pulse. Again, if you want to unsubscribe from a pulse you created, you must subscribe another owner first.
For more information about setting your email preferences, you can check out this article.