What is meant by "Content is King"?
Content is the information found on websites and blogs. Primarily in text format but content can be pictures, audio, and video. Mostly the search engines are looking for text.
Search engines are trying to provide their users with the most relevant information to the searches. So the better the engines can understand your content and the more content you have, the better they can deliver to their users.
So the more you write about a topic and the more in depth you write about a topic the more information the search engines can index from your site. So the better chance your site will be returned in a given search.
The catch is that the search engines want your website to written for people not for search engines. They will also penalise you for having duplicate content as many other websites.
Pictures can not be understood by search engines. So the old adage that a picture is worth a 1000 words doesn't hold true for search engines. Use alt tags in your html to tell the search engines what the picture is about. Then it will count as content. You should also have a caption that your readers can see describing the picture.
Audio and video are a bit more problematic. Again a descriptive paragraph about each will help your content.
Content should be original and ideally written by you. New information is important. You should keep updating your content as you learn new things. Fresh content keeps the search engines coming back often to see what is new. This will keep you up dated in the rankings.
Without the actual content that people can sink their teeth into you get a web page that reads as "buy this product" over and over again. Not useful to people.
In part this is where social book marking comes in. The more people like your content the more readily they will bookmark your site. Sales pages rarely get book marked except by the sellers.
Research your niche area in depth and provide real information which translates into quality content for people to use.