Merge Tags are different field settings that can be used across a document (like field holders) but instead of using data from your database, they replace fields with data that you set at document level.
Settings applied to merge tags are automatically applied when scanning documents for these tags.
For example, you might have a document which is the same every time you use it, except that the date and time might need to be different- such as an appointment letter.
You could place field holders in your document for <> and <