With this feature enabled you will be able to use dynamic content. Dynamic content allows you to send relevant content that resonates with subscribers on an individual or group level.
1. Navigate from the home menu to Manage.
2. Go to Lists.
3. Then List Settings.
4. Scroll down to the message composition section.
5. Select Yes for Enable dynamic content within author page?
6. Scroll to the bottom of the page and click Commit Changes.
Now that you've enabled this option, you will see the below drop-down menu on the Create Message page (Authoring > New Message). The option to add dynamic content to your message will appear just above the WYSIWYG editor: