An “Office Action” is a document written by an examiner of the U.S. Patent and Trademark Office in connection with your trademark application.
In the Office Action, the examiner will often raise "informal" matters, such as the need to provide a better description of your trademark, modify your list of goods, etc. These types of issues are usually easy to resolve, and your application will be approved once you properly address such informal matters.
A much troublesome scenario is when the examiner issues a "substantive" refusal, for example refusing to register your trademark on the basis that your mark is "generic" or is "likely to cause a likelihood of confusion" with a previously registered mark. Substantive refusals must always be treated as a very serious legal matter. Unless you have extensive legal experience dealing with trademark applications, I would not recommend you to attempt arguing against such refusals on your own. You should hire an experienced trademark attorney to assist you.
You must respond to the Office Action within six (6) months. If you fail to respond timely, your application will be abandoned.