Merge fields in the system are surrounded by three exclamation points (!!!). Whenever you see a merge field make sure you DO NOT edit the text between the exclamation points. The system uses these merge tags to populate the correct data when the email is sent.
Example Merge Fields
!!!ContactFirstName!!! -- This merge field will be replaced by the email recipient's first name when sent.
Where They Can Be Found?
Whenever you're editing an email inside the system you will have the option to use the merge fields as you please. You will see a box on the right side of the page with a list of all the system merge fields with the option to insert them into the message body.