A lot of documentation refers to the term... "Parameters" when speaking of templates.
Template parameters are variables you create that are used to pass in information to prepopulate areas of the document or to specify the information needed to send the document (such as the contact information of signers).
When using the DocumentNOW API to start a document, parameters that are required for the successful submission must be passed along in the submission.
<parameter name="FirstName" value="test" />
It is also possible to use templates online without making an API call. When doing so, you will be prompted to provide values to fill in the parameters.