Content optimization means different things when people talk about optimizing for people vs optimizing for search engines.
Optimizing for people means making the content relevant, high-quality, and making it so your users are "persuaded" to take a certain type of action (like sign-up for a newsletter, to buy something, etc).
Optimizing for the search engines is more "technical." Sure you still have to make sure the content is relevant, but you also want to make sure your title has the keyword you are targeting in it, use headings for important points, bold and emphasize other important parts of the article, etc.