In Plain English
Internal links are like doors connecting related rooms inside the same building. They let visitors move to a useful next page without starting over, and they give crawlers routes through the site. Descriptive anchor text explains what lies beyond the door better than vague wording such as “click here”. Good internal links belong where the destination genuinely supports the current task; they are useful connections, not decorations inserted to satisfy a quota.
Why it matters
Thoughtful routes improve navigation, support discovery and reveal relationships between pages. Important pages that have no incoming links are harder for people and crawlers to find.
Placement affects usefulness. A contextual link beside the idea it expands is easier to understand than an unrelated list hidden in a footer. Site navigation, breadcrumbs and curated related links can all be internal links, but they serve different jobs. Periodically check important routes after URLs change, and update broken destinations. A page should not have to depend on a site search box or a sitemap as its only route from the rest of the website.
Example
A lesson about search intent links to the related dictionary definition and the Page Checklist.
Common misunderstanding
Adding large numbers of repetitive internal links does not automatically improve rankings. Relevance and usefulness matter more than mechanical volume.