A letter or protest is a filing made by a third party against a proposed trademark which they believe should not move to registration. These letters are filed with the Director of Trademark Examining Group. This is generally done if a party believes registration will cause likelihood of confusion with one of their existing trademarks or if the proposed trademark is generic. Such a letter has to contain evidence which demonstrates that the protest is valid. If this is filed prior to publication, then a determination will be made as to whether the examining attorney may use it during their considerations. If filed after publication, then a determination will be made as to whether there was an error during examination and if jurisdiction should be restored to the examiner. If you are the one opposing a proposed trademark it is important to file an extension of time to oppose as merely filing a letter of protest will not halt the process. Letters of protest are a great way to protect your trademark, however receiving one for a mark you are attempting to register can cause difficulties and delays.
This blog is not legal advice and is not specific to your application. You should always consult an attorney.