We speak of a deep link when a link is placed to a subpage of a website. So there is no link to the main page, but to a 'deeper' page within the same site. A sub page is easily recognisable by the "/" added to the URL. There may also be further branches for these pages. But what exactly is a deep link? What is it used for? And what is the function of these links for the findability of your website?
When we look at the theory behind the wide web, we can say that there is no difference between the pages on the web. All pages are made visible to the user/reader through HTML. In theory, that is. In practice, however, this is different. As webmaster or reader you probably attach more value to certain pages. For example, the homepage is the most important page for many websites. At the same time, it is also the page most often referred to in order to attract visitors. So there is a good chance that the homepage of a website has a higher quality score.
It is not the intention that a deep link is bent for the visitor or Google. In some cases, however, you can try to hide pages. You often see this with news websites. They often have a selection of premium articles. Only members can read these articles. It is therefore not the intention that a direct link to these pages also directly exposes the information.
In the past, there have been many sites that have abused a deep link. For example, by using the content of other providers for their own interests. This, however, is seen as misuse of the information and may result in a heavy Google Penalty. In the worst case, Google may even decide to remove your site or increase your spam score enormously. Adding links to your own site with information to third parties is only allowed when it is made clear from whom this information originates. These links may not be abused. Certainly not when you do not want to jeopardise your own SEO position.