A messaging broadcast is a campaign sent via the Marketing Tools. It can be an Email sent to a group of supporters, an SMS, or a combination of both. It can also have conditional content or split tests.
Creating a new broadcast
To create a new broadcast, go to Marketing Tools > Messaging > Broadcasts.
This will list your existing broadcasts, if any. To create a new broadcast, you can either copy an existing broadcast or create a new one.
Copying an existing broadcast
This method copies across everything in an existing broadcast, including the message content and the audience query. This can therefore be a quick way to create something similar again. To copy an existing broadcast, click the copy icon 
next to the item in the list.
...
Create a new broadcast from scratch
To create a new broadcast from scratch, click the orange Create Broadcast button.