In practice
Diagnose it by intent, not by design: the usual cause is a page that answers a different question from the one asked.
Put the answer above the fold. Pages that make a visitor hunt for it lose them back to the results.
Why it matters
It is the clearest behavioural signal that a result did not satisfy the query.
Unlike bounce rate, it is measured against the alternative — the searcher chose a competitor instead.
Related terms