Dynamic Field Value Substitution means dynamically substituting the email id of the person
in the role specified in the 'To Field' of the email setting.
For example consider the case where email should be triggered to the submitter of the workflow.
To access the 'Mail Settings' screen, follow the below link:
'Control Centre->Workflow->Emails' icon under the sections/groups.
In the 'EMail' Settings screen, click on the 
symbol against already configured email.
In this case the word 'submitter' will be used between 2 hashes before and after the word. eg: ##submitter## in the 'To Field'.
or Click the 'Add Mail' button and click on the 
symbol and add the word 'submitter' beween 2 hashes before and after the word.
The below screen shot shows how this feature is used in the cflow application.
Click here to go back to see the other options of Email customization.