A trademark applicant will receive an Office action if his trademark registration application is rejected by the U.S. Patent and Trademark Office. It is an official document written by an examiner and it usually includes the reasons of rejection and a list of requirements that the application needs to meet in order to be accepted for reconsideration.
If you wish to continue registering your trademark, you have to answer within six months from the mailing date of the office action letter. If you don’t want to continue, all you have to do is not answer, and your application will be abandoned.
A non-final Office action presents an issue identified in your application for the first time. A final Office action is issued after you have answered to a prior Office action and failed to resolve the presented issues.