An opposition window is a phase of the trademark registration process during which the owners of previously registered trademarks can challenge the applicant's mark. Since any new mark can unintentionally infringe on someone else's already existing trademark, the opposition window allows trademark owners to exercise their legal rights.
Owners usually raise opposition if they believe the new mark is confusingly similar to theirs. The opposition process then varies between countries, but generally, both parties can either reach a solution amicably (such as by signing a co-existence agreement) or a resolution will be reached by the IP office. It is possible for the applicant to defend their mark successfully.
The length of the opposition window also depends on the country but is usually 2-3 months. The US has the shortest opposition period lasting for only one month. If no opposition is raised during this time, the trademark will be successfully registered. If an opposition is raised and the proceedings start, they can last another few months.