In laymans language, it is the links that are tagged to the pages in the same website.
Uses of link building.

They allow users to navigate a website.
They help establish information hierarchy for the given website.
They help spread link juice (ranking power) around websites.

Benefits of Internal Linking

More site navigation= more user engagement.
Increased page views= more revenue.
More user responses and article interest.

Too much internal links makes the article look spammy and thus increase the exit rate and bounce rates. So be aware.

Process of linking your website page to another page with the same domain, that directs any user inside the website is internal link building.

Internal links are the links that are traverse you to one page to another page on a same domain.

Internal links are links that go from one page on a domain to a different page on the same domain. They are commonly used in main navigation. These type of links are useful for three reasons: They allow users to navigate a website.

Meta Description,Sitemap,URL Structure,Image Optimization,Update Content Regularly,Keyword Density

It is an On-Page seo technique. In this we simply link the content of one page to another page making hyper-links. When one user clicks on that link, he/she moved to another page.

Meta Description
Sitemap
URL Structure
Image Optimization
Update Content Regularly
Keyword Density.

An internal link is a type of hyperlink on a webpage to another page or resource, such as an image or document, on the same website or domain.

Internal links are links that go from one page on a domain to a different page on the same domain. They are commonly used in main navigation. These type of links are useful for three reasons: They allow users to navigate a website.

Internal Links are hyperlinks that point at (target) the same domain as the domain that the link exists on (source). an internal link is one that points to another page on the same website.

Internal hyperlinks are links that go from one page on a website to a special web page at the identical domain. They may be generally used in fundamental navigation. Hose type of hyperlinks are useful for three reasons: they allow customers to navigate a website.

Internal linking refers to any links from one page on a domain which lead to another page on that same domain. This can refer to the main site navigation, like the links you see above this article to our sections on SEO, PPC etc.

Internal link building referencing is a type of craftsmanship; the "specialty of SEO". This is in a way like building backlinks to your blog without anyone else blog. It does everything that a backlink does from passing connection juice to bringing more activity and upgrading internet searcher rankings.

Internal links are links that go from one page on a domain to a different page on the same domain. They are commonly used in main navigation.

Internal linking is linking from one page on a blog to another on the same blog.