An internal link is a hyperlink that is a reference or navigation element in a document to another section of the same document or to another document that may be on or part of the same website or domain of the internet.
Links are considered either "external" or "internal" depending on perspective. Generally, a link to a page outside the same domain is considered external, whereas one in the same domain is considered internal.
However, these definitions become clouded when the same organization operates multiple domains functioning as a single web experience, e.g. when a secure commerce website is used for purchasing things displayed on a non-secure website.
Similarly, a blogging website might have thousands of different blogs, in which context one might view a link as "internal" only if it linked within the same blog, not to other blogs within the same domain.
Make a header linkable:
<h1><a name="seealso">See also</a></h1>
then link to it:
<a href="#seealso">Go to the see also section</a>