SEO comes in two different flavors that will affect the ranking of your business' website: off-page SEO, which mostly boils down to the links coming to your site from other sources, and what kind of anchor text they use, and on-page SEO, which takes into account the content on your page, how it relates to content on your website in total, how relevant it is to the theme, what kind of text is in the links going out of your site and the sites they link to and the density of keywords on the page.
On-page SEO is important for search engines to determine what to rank your business' website for. You must have clearly defined header tags, meta keywords, and a website that is easy to crawl for search engine spiders. Off-page SEO focuses more on the outside sites that are linking back to yours, and a search engine's algorithm looks more at how related the content on them is to your website, the position of your link, and the anchor text used in the link to determine how much the link is worth to them.
On-page factors are sometimes underestimated because of the fact that even a massive amount of highly unrelated links can make a page rank for a specific term- the infamous Adobe ranks for the keyword “click here” despite having no header tags targeting the phrase, few instances of the phrase across the site, and the keyword isn't bolded or emphasized in the content or used as anchor text for links out etc. Nonetheless, on-page SEO helps to determine the way your website ranks in search engines, and pages that are highly optimized for content in the way that the following articles will explain rank far easier for keyterms that they are optimzed for than other pages that are not optimized for them, but also targeting them. There are also some off-page "prohibited" SEO techniques that can be banned or panalized by search engines e.g. Google.