If your trademark application passes muster with the Examining Attorney assigned to your case, you will receive a Notice of Publication.  During the publication period, which is 30 days long, third parties can file a formal opposition against your application or request an extension of that 30 day period.  If you receive notice that someone had requested an extension, they are likely will reach out to you about the opposition or coming to some understanding or amendment of your application. And if you receive such a notice, or worse, notice that a formal opposition or cancellation proceeding has been filed, you should discuss your options with an attorney.