Search analytics is the measurement and interpretation of on-site query data — what visitors search for, what they find, and what they do next.
The core report set is small: top queries by volume, top queries by revenue, zero-result queries, low-click-through queries, refinement chains, and search versus non-search conversion. Nearly every actionable search decision comes out of those six.
Its second use is not about search at all. The query log is unprompted, unbiased demand data: it reveals products to stock, content to write, terminology customers actually use, and questions the site fails to answer anywhere.