SEO is a term or strategy to show your online presence on famous Search engines especially on Google so that visitors can access your content by searching keywords on search engines.
SEO is key factor to improve your site ranking in Search Engines especially on Google. There are 2 main types of SEO, 1) On-Page SEO, 2) Off-Page SEO.
For more information you can read this article from SearchEngineLand. And for SEO Tools you can visit SeoToolsCloud.
One main thing you should keep in mind that doing SEO is not a easy task, it requires skills and money. Wish you best of luck to understand SEO.
SEO is all about making your site handsome. Work smartly and be lover of google. SEO is to make a affair with google so that he would give importance. So, obviouly it is ranking factor. For good seo you should have a super content and great witnesses. Content makes you great.