Search Engine Rank is relating to page rank. When you search any keyword on a search engine then its database display thousands of results in a search engine depending on page rank. A Page Rank is measured by the position of the webpage is displayed in the search engine results.
If a search engine is putting your webpage in the highest rank on a search engine results, it means your webpage is rank higher position on a search engine.
SEO is a process to design and develop a website in such a manner so that website always ranks on the first page of search engine results.