Internal Link
A link from one page of a site to another, present in the HTML response rather than added by script.
What it means
Internal links are how a crawler finds the rest of a site and how it works out what a page relates to. They also carry meaning: a link from a page about inspections to a page about structured data says those subjects are connected.
The part a short definition leaves out
Zero internal links in the raw HTML usually means the navigation is being built client-side. That is not a content problem and no amount of adding links in the editor fixes it, because the editor output is not what the crawler receives.
See whether this is costing you anything
The inspection reports on this signal directly, alongside thirty-seven others, and tells you whether it is a quick fix on your site or something deeper.