The more SEO you can implement for the various pages of your website, the more organic traffic it will accumulate. A proper SEO strategy employs methods that occur both on and off the website.
Both are important. However, ON-SITE SEO is something you can play around with in the beginning of your site launch. This is the content creation, formatting and site structure aspects of your site. OFF-SITE SEO is all about the promotion of your content, such as social media, link building and guest blogging on other sites.