Internal links are hyperlinks that link to the target, within the same domain as the domain where the source link exists. In simple terms, an internal link is one that points to another page on the same website.
Generally speaking, internal linking refers to any link from a page in a domain that leads to another page in the same domain. This can refer to the main navigation of the site, as well as links that show different sections of the website. It also refers to links within articles with associated content.
In general, a link to a page that is outside the same domain or website is considered external, while one that points to another section of the same web page or to another page of the same website or domain is considered internal.
The Internal Linking is a practice SEO that used properly can offer good results to improve the positioning of the most relevant pages of a website, since with them you can configure a web architecture in which authority flows to the most important pages of the site.