Patent is something that an inventor tries to get to have legal protection of using his own invention. It is sort of like a deed for a house. The inventor submits an application, in the "claims" section the owner/inventor would declare in great detail what the invention covers, kind of like a deed should say exactly where the edges of the property are in order to identify to whom the land belongs. Once the patent application is approved, it gives the owner legal right of the invention. When other people try to make the invention that is declared in the "claims" section, they will need to get authorization from the patent owner/inventor.
So a patent examiner determines if an application can be approved. We focus on the "claims" section and compare the invention declared in the "claims" to inventions that had been described/made in the past. So we search past patents, library databases, scientific journals, etc., to find similar past inventions. If the "claims" declare an invention that is unique and novel compared to old inventions, then the inventor gets approval to receive a patent. Each patent examiner looks at one or a few types of patents, like I look at simple medical devices but other examiners may look at chemical compounds, engines, electronic devices, etc. I was qualified as a patent examiner because of the engineering classes I took. In this job, we end up doing more of legal writing than real engineering. We just need basic engineering principles so that we can understand the invention in the application (and the old inventions that we have to look through). The real work of this job is in applying the law when we approve/reject patent claims.
Hopefully this paints a slightly clearer picture of this profession for you!