On-Page SEO vs. Off-Page SEO

 On-page SEO covers anything you can do on a webpage (or internally) to improve your rankings. 

Off-page SEO covers anything you can optimize outside of your site (or externally) in an attempt to boost your rankings. Backlinks are arguably the biggest off-page SEO factor. Other examples include social media and PR.

Both are important components of any good SEO strategy.

But you have more control over on-page SEO factors. So focusing on those is a good place to start.