It's called King of SEO because it's the most important factor in determining your search engine ranking.
Basically, you need to write content that is:
-Easy to read
-Well-structured
-Varying in length and tone
-Informative and interesting
Great content helps build trust between the reader and author (or brand). It shows them that you know what you're talking about and care enough about their needs that you'll spend time creating something truly helpful and meaningful for them.