What is link building?
In a colloquial and simple way, link building is the art of getting other sites to link your page or section, or internally on your page place links between different pages or sections. That is to say, that one of your entries is present on a web page different from yours, in this way the visitors who are browsing in a site that has linked you, can end up entering your web page.
Because it is important?
Getting other websites to link your content is important because it helps your portal to position itself in the search engines. What you achieve with this, is that search engines trust your website. This is what is known as the domain authority. The number of incoming links is one of the factors evaluated by PageRank and therefore improve your authority on the web.