The OffPage-Optimisation is one of the two pillars of Search Engine Optimisation. You can differentiate between the OffPage- and OnPage-Optimisation (also called OnSite- and OffSite-Optimisation).
The OffPage-Optimisation encompasses all the external factors of a website. Usually, this means the backlinks, also called external links or just links.
Backlinks are still one of the most strongly weighted ranking factors and, as such, they are a necessity for a good ranking within the organic search results. Links are rated and weighted differently by the search engines. That is why the number and quality of backlinks are important factors.
You will often hear about backlink building when it comes to the OffPage-Optimisation of a website. This refers to your own active engagement to generating links and backlink partners. To reach this goal, you will often use a mix of different link building strategies, like a Linkbait or Content-Syndication.
The backlink profile of any domain and single link metrics can be analyzed and evaluated in detail using the SISTRIX Toolbox: