An office action is a communication from the Trademark Office. It may be an inquiry or it may be a communication of an action the Trademark Office has taken. Office actions can be considered “substantive” or “non-substantive.” A non-substantive office action is usually a technical correction to the application or a procedural request from the Trademark Office, and they do not require legal analysis.
A substantive office action is usually but not always a rejection of some sort – for instance, the Trademark Office might send an office action because it has determined that another trademark registration presents your application from being accepted. There is no official definition of what constitutes a substantive or non-substantive office action. To a certain extent, it is an arbitrary distinction.