Before issuing a trademark registration, the USPTO will “publish” the underlying application in case others want to “oppose” registration. There is a 30-day window for filing an opposition, but it can be extended upon request.
Where does the USPTO publish applications, you may ask? In the Trademark Official Gazette, of course. The Official Gazette – literally the OG – is no longer printed, but is available online at the USPTO. Many trademark monitoring services will keep tabs on the OG to see if potentially conflicting marks are being published.