What it records
PHP fatal errors, warnings and deprecations, and database errors. Each is grouped by its normalized message and attributed to the plugin, theme or core file that raised it.
Why grouping matters
A single defect can fire thousands of times. Detective keeps one row with a count instead of a flooded log, so the signal is readable and the truly noisy problems stand out.
What to do with one
Open the row for the file and line. If it started right after an update, that update is the suspect until proven otherwise. Deprecations are early warnings of a future PHP break; a sudden rise in warnings deserves attention. Copy the full message, file and line when you report it to a developer — that is exactly what they need.