When you apply for a trademark, the assigned examiner assesses your application. If they discover a problem preventing the trademark's registration, they issue an office action and stop the registration process.
The reasons can range from mere formalities to more complex substantive issues with the mark. Simple administrative issues may, for example, include an error in applicant details. On the opposite end of the spectrum, we can find dismissals due to a perceived lack of distinctiveness or likelihood of confusion with an already registered trademark - but even these can sometimes be overcome.
If you've received an office action, the document itself should state the reasons for dismissal as well as the proposed remedy and the deadline for responding.
If you are not sure how to read a US office action, you can find the most common issues listed in our Trademark Q&A category dedicated to office actions. You can also approach an attorney who can help you understand the issue, assess your chances of overcoming the office action, and craft a proper response.