Stable Identifier
A durable URI given to a node in structured data so other nodes can reference it instead of repeating it.
What it means
Without identifiers, each block of markup is an island describing something in isolation. With them, an Article node points at its own WebPage node, which points at the Organization that published it, and the whole set describes one connected thing.
The part a short definition leaves out
The identifier only helps if it stays the same. Regenerating identifiers on every build, or tying them to a URL that changes with every redesign, gives you the syntax without the benefit.
Where this comes from
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.