Let’s say you are creating a page about elephants, you would write probably ten or twenty articles on the subject of elephants. Putting links between these ten or twenty articles is important for SEO. For example, let’s say one article deals with the height of the elephant, and the other deals with the weight of the elephant, you may want to put a link between these two articles, so people reading one can quickly jump to the other. This is called “internal linking” or linking between pages on your own site.
When you do internal linking, your site is perceived by the search engines as more hand-crafted. It shows that a lot of thought and effort has been put into it. And if a lot of thought and effort has been into it, then it is probably something of a lot of value. The more internal linking you do, the better your site is perceived in the eyes of the search engine.
When the search engine spider crawls one of your article it looks for links in the content. If there are links to other pages on the same site, it jumps to it and assesses the relevance of this other article to the one from which it was linked. The relevance of interlinked articles to each other is very important too. Why would you want to link to something totally unrelated? When you hook your reader on a particle subject or idea, you want to feed them more of that to keep them interested and make them more educated on the subject while they’re at it.
Articles that are not closely related to each other should not be linked and it is not good for SEO purposes.
The whole idea is that search engines want to provide value to their users and this means as much “relevant” content as possible.