According to Google:
SEO is an acronym for “search engine optimization” or “search engine optimizer.” Deciding to hire an SEO is a big decision that can potentially improve your site and save time, but you can also risk damage to your site and reputation. Make sure to research the potential advantages as well as the damage that an irresponsible SEO can do to your site. Many SEOs and other agencies and consultants provide useful services for website owners, including:
•Review of your site content or structure
•Management of online business development campaigns
•Expertise in specific markets and geographies…
…If you’re thinking about hiring an SEO, the earlier the better. A great time to hire is when you’re considering a site redesign, or planning to launch a new site. That way, you and your SEO can ensure that your site is designed to be search engine-friendly from the bottom up. However, a good SEO can also help improve an existing site.