A trademark opposition is a legal challenge to the right to register a particular trademark. There are thousands of trademark oppositions filed every year, which certainly sounds like a lot, but is actually quite small relative to the number of trademark applications filed annually.
A trademark opposition is initiated by electronically filing a notice of opposition with the Trademark Trial and Appeal Board (TTAB) through the ESTTA system and paying the required fee. The notice of opposition must be filed within 30 days of the date that the trademark application is published for opposition, unless a request for an extension of time to oppose is filed with the TTAB (which is usually automatically granted). If you unfortunately miss the opposition window, then you’ll need to wait until the trademark actually registers and then file a trademark cancellation against the registration.