Off-site SEO is anything you can do to get traffic from different channels as links, social media, social bookmarking and more.
On-site SEO is optimization of different parts of your website, including content
In this case on-site SEO is more important to Google and I recommend to focus on it more in the first place. You can start improving on-site SEO by adding title tags and meta descriptions with your main keywords in it, header tags, that would make your content an easy-read piece, and alt text for images to describe images on the web, so if the image was searched in Google, it would lead to your content.
External and internal links
Off-site SEO is an amazing tool to promote your website and improve its ranking by backlinking, however, while focusing on the external links, we often forget to do internal linking. Google uses both – internal and external – links to set the value of the website and if there are a lot of links to a page, Google will decide that this page is important and will rank it higher. Internal links is a way to keep your visitors on your website and to improve the Google ranking.
External links that are leading visitors to your website are also very relevant here but it is crucial not to forget the quality of those links. It is important to keep them relevant and included in an authoritative site, links from low-quality domains won’t do any good here.
It is important to know that Google is not identifying Twitter or Facebook shares the same as the links from the authoritative pages. However, all the higher rankings on Google are the ones who are getting more social shares than others. In this case it is proven that social media presence is also helping to get a higher rank on Google Search Engine, still, it is important to engage with your visitors and make the content easy to share.
To keep your content shareable, add visual elements that would make people click on, use hashtags to make your posts more discoverable and know your audience – from this you will have to decide what social platform to use and what is the best way to communicate.