Internal links are rather easy to understand. The idea of an internal link, is to keep your current audience on your site for a longer period of time. In hopes to sell or educate them on your product.
In order for your site to have a lot of internal links; your site must present a lot of content. For example, if your site offers a specific service such as SEO. Having articles, or blogs, about SEO would be perfect content to link to internally. Setting these links, to these deeper pages, results in a an overall SEO boost for your site. Ideally, the best placement for an internal link is within a body of writing. This results in your audience coming across the hypertext of the link more naturally. Rather than placing a random link at the bottom of a page. All in all, the more content the better. Internal linking will take your website to the next level!