Lots of website owners trying to improve their SEO heed the advice that backlinks are important and then put 100% of their efforts into creating those links from every website and social media platform they can find. They overlook that the links they have within their website and how they are structured are also important for SEO, and they play a role in helping visitors who visit that site too.
An internal link acts in much the same way an external link does in that when it is clicked; it will take the person who clicked it to a different location.
Whereas an external link will take them to a different website, an internal link will either take them to another page within the website or to another part of the page, or it could be used to allow the visitor to download something, such as a document file.