This feature encourages engagement among the organizer & participants and helps in making the session and event more interactive.
There are two ways to broadcast your message:
At Event Level
At Session Level
Anytime during the event, Admin/ Host may write message and broadcast to the target audience (attendees or panelists).
In case, you select the former option (Broadcast to public) then it will include speakers as well and the message would appear on their screen too.
There are two ways to make the announcements:
From Dashboard
Go to the Host Dashboard
Go to Announcement
Select Whether you want to broadcast message at Event Level to the entire participants or only to the people in Session

For Session Level, Select the Session.
Type in your message, Click on Broadcast

The message would be displayed on the bottom left corner of the screen.
From Room Settings
You can also Broadcast message at both Event or Session Level by navigating to Room Settings(bottom right) > Broadcast

Note: You can also send mass message to your participants via Organizer Dashboard anytime, before, during or after the event. For this, kindly refer to this article