An “Examiner” — in the context of patent applications — is an employee of the USPTO, usually with a scientific or engineering background, tasked with reviewing patent applications for eligibility, including novelty and non-obviousness. Fundamentally, they act as an ‘advocate for the public,’ working to ensure that the public receives a disclosure of an invention that is both novel (new) and non-obvious in exchange for the period of exclusivity that a patent grants to an inventor. As such, they:
… serve as a judge on patentability with respect to inventions claimed in a patent application under conditions for patentability set forth in Title 35 of the United States Code …The Role of the Patent Examiner
In other words, an Examiner is the person at the USPTO tasked with reviewing your patent application, and working to make sure that the claims do not overlap what is already publicly known. They often issue one or more rejections of your claims (as well as objections and informative actions) during the prosecution process. They may also allow one or more interviews with your patent professional, where they will discuss the claims and hopefully work to find an area of agreement that may expedite allowance of one or more claims.