Internal links are a part of a link building campaign. An internal link is nothing less than a link to another page within your website. Internal links should be a very important part of your online marketing strategy. This is because with internal links you ensure that the search engine will better understand the structure of your website.
The nice thing about internal links is that not only does it benefit the search engine, but also the visitor. If we look at wikipedia, many internal links are also present here they do that to better explain certain concepts on a separate page.
For example, within this article we place a link to the link building page. There we explain in detail what the concept of link building is.