There are many SEO techniques that are always very important, we need to know about SEO onpage and offpage. With each part, example Onpage, there're many factors such as title, description, meta tags, keywords density, sitemap, robot.txt, ...we need to learn and practice all.
With SEO Offpage we also have many factors such as domain keywords, link building, social signals, CF, DF,...
I hope this helps.