Mail merge is a process of merging or importing data from a .NET object, also known as data source, to a DocumentCore instance, also known as template document.
Binding between data source and template document is provided
by Field class whose FieldType property is MergeField (usually called a merge field).
The method GetInstructionText() returns text that refers to the name of the property or column in data source (usually called a merge field name), and in mail merge process, that Field instance will be replaced by actual data returned from the data source for the given property or column name.
Here you will find interesting examples to understand Mail Merge process: