A letter of protest is a filing at the Trademark Office that allows a third party to provide information to the examiner about someone else’s pending trademark application.
Let’s say your business becomes aware of someone else applying for a trademark that you think is confusingly similar to yours. Or maybe the application is for a common term in your industry that you don’t think that person should be able to monopolize as a trademark. While you might be able to wait to oppose the trademark later in the process, a letter of protest may be more effective.
The USPTO explains that it only considers letters of protest that include “a specific, relevant legal ground (i.e., reason) for a USPTO examining attorney to refuse registration or issue a requirement in a pending application.” Those reasons include that the mark is likely to cause confusion with another registered mark (or one in a previously filed, pending application) or that the mark is merely descriptive or generic. There are other reasons, too.