Structured data21 is code that you can add to your sites' pages to describe your content to search engines, so they can better understand what's on your pages. Search engines can use this understanding to display your content in useful (and eye-catching!) ways in search results. That, in turn, can help you attract just the right kind of customers for your business.
QUOTE: “high quality content is something I’d focus on. I see lots and lots of SEO blogs talking about user experience, which I think is a great thing to focus on as well. Because that essentially kind of focuses on what we are trying to look at as well. We want to rank content that is useful for (Google users) and if your content is really useful for them, then we want to rank it.” John Mueller, Google 2015
QUOTE: “The score is determined from quantities indicating user actions of seeking out and preferring particular sites and the resources found in particular sites. *****A site quality score for a particular site**** can be determined by computing a ratio of a numerator that represents user interest in the site as reflected in user queries directed to the site and a denominator that represents user interest in the resources found in the site as responses to queries of all kinds The site quality score for a site can be used as a signal to rank resources, or to rank search results that identify resources, that are found in one site relative to resources found in another site.” Navneet Panda, Google, 2015

QUOTE: “The duration performance scores can be used in scoring resources and websites for search operations. The search operations may include scoring resources for search results, prioritizing the indexing of websites, suggesting resources or websites, protecting particular resources or websites from demotions, precluding particular resources or websites from promotions, or other appropriate search operations.” A Panda Patent on Website and Category Visit Durations

The SEO starter guide describes much of what your SEO will do for you. Although you don't need to know this guide well yourself if you're hiring a professional to do the work for you, it is useful to be familiar with these techniques, so that you can be aware if an SEO wants to use a technique that is not recommended or, worse, strongly discouraged.