Definition
A knowledge graph is a structured store of entities and the relationships between them. Public graphs such as Wikidata are read by many systems, which is why an entry there can influence how a brand is identified far beyond the site that hosts it.
Why it matters
It is one of the few places where a small brand can state its own identity in a form that machines already trust, without waiting for coverage it cannot control.
How to measure it
Check whether a public entry exists, whether the facts in it are current, and whether it links to your official domain and profiles. Correcting a stale entry is usually higher leverage than adding another page to your own site.