An internal link is a hyperlink that points to another page on the same website. Internal links are how a user navigates a website.
A good internal linking structure is crucial for providing a good user experience. It's also crucial for helping the search crawlers determine the hierarchy of your pages and to find all the pages to index.
A high number of internal links pointing towards a page will tell the search engine that that page is important. External links will also give this page extra importance and authority. Good SEO practice is to use internal links to pass this authority on to pages further down the hierarchy to give them a little boost.