Merge fields let you insert field
data from Listing or Contacts into
Emails,
SMS messages,
letters, and
newsletters. This is particularly helpful for communicating in bulk with your database, and it also allows the system to send dynamic
emails automatically using the
workflows module.

When using merge fields you should always test what you are sending to ensure that the merge fields you have selected have data against them
How do I insert merge fields?
Merge fields can be inserted into Emails, SMS messages, letters, and newsletters. To insert a merge field, simply click the "Merge Fields" button when creating or sending any one of these items. This will open the "Merge fields helper," from here you can browse the fields that are available to you and click the "Insert" icon to insert that merge field into your document.
What happens if the data in the merge field is not available?
Sometimes you might be sending an email to a lot of contacts, but not all of the contacts have the merge field data against them. In this case, nothing would show where that merge field is inserted in the document.