Merge fields allow you to insert user-specific information in an individual activity, while using a mass activity wizard. For example, if you use the Mass Email Wizard, you could make the first line of the email be "Dear #FIRSTNAME#". If the receiver of the email was John, the actual email that John received would read, "Dear John". Other commonly used merge fields include #LAST NAME#, #PREFIX#, #SUFFIX#, and #REQUESTAMOUNT#. All the possible merge fields are listed under the "Insert Merge Fields" option in the Activity Wizards.
Note: Merge Fields are currently only available for the Mass Email Wizard.