An Office Action is an initial refusal by the USPTO to register your mark. Office Actions can be based on minor issues, like not providing all the required information in an application, or major issues like a determination that your mark is confusingly similar to another registered or pending mark.
Office Actions must be resolved before your mark is allowed to proceed to registration. The USPTO generally gives you 6 months to respond to an Office Action. Failing to respond can result in your application being deemed abandoned. In many cases, it helps to consult with a trademark attorney who can review the Office Action and determine the most appropriate way to respond.