What is the most important think about pro level SEO?

SEO tasks are devided into two main parts: internal optimization (eliminating page errors, writing metatags, determining keywords list, and other things specified in the previous reply) and external (backlinks). Either of them is very important. Building high-quality backlinks profile is a long, but resultative process in your SEO strategy.
 
To learn more you need to take some good courses and check our blogs of Neil Patel and Plerdy.

Here are some of things that play a major role in SEO

• Title and URL
• Keywords 
• Content with keywords 
• Content with image name and alt tag with title and keywords
• Size of the content, authortative (1600 words + )
• Page load speed and rendering time
• Mobile visibility and performance (responsiveness) and page load time
• External links to the URL (inbound). Higher ranking domains inbound links are better.
• Sharing on social media
• Time spent on the page by visitors 

Hope it helps!