A method using internal links within the website content. Basically, a deep link is any link from one web page on a website to another web page on the same website. This is used in calls to action or within articles, where a link is used to lead the visitor to a page with more informative content.
It is an important SEO method to spread link equity to your web pages and prolong visitor dwell time and engagement. Also, you will probably have a different, but topically similar set of keywords for each page and linking between them increases overall page relevancy.