what is major part of seo..?

There are basically two types of SEO techniques - on page and off page.

On page seo refers to the process that is followed on the website itself. Some of the tasks involved are:

Keyword research
Meta tags and title
Meta description
Robots.txt file
Sitemap generation
Content development
Image optimization

Off page refers to the link building and promotion of your website in search engines. Some of the most effective off page activities are:

Guest posting
Article submission
Press release submission
Social bookmarks
Classified ads
Infographics submission
Social media optimization

In SEO major part is
Onpage optimization and Offpage optimization both are very important,
keyword research and choose best keywords are also important
do onpage for seo friendly after do offpage

SEO has 2 sides 1) On Page 2) Off page. Both are important equally. Because if something lack there you can't rank your keywords to google or another search engine.

On page SEO is the main part of SEO.

Content/Creative. This is the most important part of SEO. Content should be at the core of your SEO strategy. Whether it's articles, images, videos, infographs or a collection of all of these, the more relevant and helpful your content is, the better your chances of it being found on the web.

Accurate and perfect On page is important for any website SEO, after that Off-page of the website is important.

On-page SEO is the main part of SEO.

On-page SEO and Off-Page SEO is the main part of SEO.