The U.S. Patent and Trademark Office can deny your application for federal trademark registration for any of the reasons stated in Section 2 of the Trademark Act, 15 U.S.C. § 1052.
There are about twenty. However, you can forget about most of them. For practical purposes, the main ones to worry about are: likelihood of confusion, mere descriptiveness, deceptive misdescriptiveness, primarily merely a surname, and unlawful use. These objections are summarized below.
-Likelihood of confusion. Frequently, the most tricky objection to consider is whether your mark is confusingly similar to a prior filing. This mainly means that your mark is too similar to the mark reflected in a prior-filed application or registration, in the context of the associated goods and services, such that ordinary consumers would likely, or probably, mistakenly believe that your goods or services come from the prior filer.