On-Site Optimization refers to the optimization of the website “infrastructure”:
1. Tag <title>
2. Meta tags
3. Alt tags on the images
4. Text formatting (headings, bold, italic)
5. robots.txt file
The title tag is maybe the most important here, not only for the SEO but also for the CTR (click through rate) in the SERP (search engine results page). The title of each page must be different, so that it won’t be considered duplicate content, but it should have 2-4 keywords. For example, a page title of type:
Kw1, kw2 – site name, description with kw3 in it (in my opinion the most appropriate method)
Most of the meta-tags are informative, the only ones that mater being the meta description and meta keywords. Whatever is written in the meta description will appear as title in SERP. It’s important to have one or two keywords here too.
The headings, bolding and italicizing the text matters less, but it matters. Obviously, keywords are bolded, not something else. The idea is that the search engine sees the mark-up as a bolded word, so it decides that it is more important than the rest of the page and might be a keyword.
Robots.txt is a file in the root of the site from where you can control the way that the SE crawlers index your site. You can ask them not to index certain pages with the no-index attribute or not to transfer from the authority of the site (for example pagerank) with no-follow. With robots.txt and some internal linking, you can control the total PageRank on the site and focus it only in some important pages.