After the United States Patent and Trademark Office agrees to issue a registered trademark, they publish the mark for a period of public opposition.
During this period, any person or company that believes they would be harmed by the registration may either file an opposition, or ask for an extension of time to evaluate whether they will initiate an opposition proceeding.
These proceedings are relatively rare, but if initiated they act like a mini-trial – complete with motions, discovery, and witnesses. An opposition proceeding will always create a delay in registering your mark, and will often increase the costs associated with obtaining your mark.
This process also provides a useful and necessary method for an individual or company to step in and prevent them from being injured by the Trademark Office not doing its job properly and allowing a potentially confusing or descriptive mark to be registered.