Merge fields can be used in Email Templates and Text Templates to automatically input your lead’s info into the message (such as your lead's name, the address they're inquiring about, or your name and phone number).
For example, if you want your drip emails to address your lead's by their first name at the start of the email (such as "Hi Paul!"), then you can use the "Contact first name" Merge Field.
To add a Merge Field to an Email Template or Text Template
Agent merge field / Lender merge field
You may wonder what "Agent merge field" and "Lender merge field" are in that list (%agent_merge_field_1% and %lender_merge_field_1%):
Those come from the additional free-form merge field "User Merge Field" that agents and lenders can edit on the My Settings screen. They can put any additional information into this field, for example a link to their personal website, a link to schedule a meeting, a job title or anything else.