After the USPTO Examining Attorney approves your trademark, your mark will proceed to publication.
During publication, your trademark will be “published for opposition” for a period of 30 days.
This 30-day opposition window is similar to the part of a wedding where they say “speak now, or forever hold your peace”.
During this 30-day period, any third party can object with a Notice of Opposition against your mark.
Essentially this means the third party is opposed to the USPTO giving you a trademark registration.
A Notice of Opposition is a formal objection against the registration of a trademark filed with the Trademark Trial and Appeal Board.