SEO means search engine optimization. It refers to an ever-changing array of strategies used to make web sites more search engine-friendly. As search engines update their algorithms, the strategies change to meet them.
Headings: What's important
Like your human readers, search engines use the headings on your page to figure out what the page is about and where on the page the information exists. Headings are also a great place to use the words and phrases your audience will be searching for.
Note that headings aren't merely for esthetic purposes. They're also crucial to the visually impaired. See Accessibility for an explanation on how to use headings.
Images: Letting Google read your eyes
Computers can't (yet) 'read' images in the way they can text. Alt text provides explanations that search engines can interpret. Every image should have an alt tag. See Accessibility for an instructions on how to add alt text.
Keyword lists are bad SEO
"Keyword stuffing" is the practice of adding long strings of keywords without incorporating them into meaningful content. Google recognizes this as unhelpful to users, and will ignore or punish pages that participate in keyword stuffing. All keywords and phrases should be part of the natural flow of your text.