Dynamic content blocks allow you to show different content in an email to your contacts based on their interests.
For example send a single email with mostly static content but show a specific section only to board members and another section only to staff.
How to add dynamic content blocks
For each content block in the message editor you can choose the tag or subscription your contacts need to be belong to in order to see that content.
When dynamic content is applied to a content block you'll see the following notice so you can easy tell apart static content from dynamic elements.
How to test dynamic content
Use the Preview as... option in the email editor to see the content that a contact on your list will see.