What does it mean that links are first-class citizens?

Relationships between ideas deserve as much attention as the ideas themselves.

Example: You have a note about 'spaced repetition' and a note about 'habit formation.' The link between them — that spaced repetition works because it exploits the same neural pathways as habit formation — is not a filing convenience. It is a standalone piece of knowledge. If you delete the link, you lose an insight that neither note contains on its own.

Try this: Open your note system and find two notes you believe are related but haven't explicitly linked. Before creating the link, write one sentence describing the relationship: what exactly connects these two ideas? Now create the link with that sentence as the anchor text or annotation. You've just promoted a vague association into a first-class knowledge object.