Announcements are a special type of topic that only admins can use to send messages. The announcement topic is always pinned to the top of the topics screen and visible to everyone in the group at all times. Any admin can send an announcement.
When an announcement is sent, the admin has the option to send only in Heylo, or also distribute the message by email as well. If email is selected, everyone in the group gets a notification AND an email communication. Emails are sent to any members who have signed up for the group, as well as any members invited to the group by email.