SEO company basically helps websites to rank for keywords in their niche. They do this by implementing a number of strategies & tactics to make the website relevant and authoritative for the keyword in the eyes of Google and the website users. For example, when a person is searching for dog food in Sydney, any website that sells dog food or provides information on dog food in Sydney is relevant to the search.
They do this by working on a number of factors that helps them to rank on search engine results page like on-page SEO, technical SEO fixes and backlinking techniques.