The 'Allow message modification' option enables anyone visiting the action page to edit the message before completing the Action Edition.
Because of this functionality, be sure to make your Action Message Template to be plain text - not HTML - so that people edit the plain text and the target receives plain text.
For more information see: http://www.fairsay.com/labs/ectool/docs/BasicSetUp#step-3-add-an-action-message