Search Engine Optimization mainly comprises of two parts – onpage and off-page(aka content Marketing) optimization.
When you optimize each web page in order to generate relevant traffic for that individual webpage. If the page is about web design, then you optimize the page title, description, heading tags, alt tags, content around similar theme.
It is completely opposite to the on page techniques and refers to the external links/signals coming to that webpage. The more niche relevant it is, the better and easy it is for the page to rank.
In the off page optimization; its not about the quantity but the quality of each link that's coming to your website.