The reference index contains information about all database relations in TYPO3. It is used to find references between records and check their integrity. This index should be regularly updated to ensure all references are tracked correctly.

Check: Verifies the reference index and reports any issues without making changes.
Update: Rebuilds the entire reference index. This may take a while on large installations.

You can also run the check as a shell script using CLI if the processing takes longer than the PHP max_execution_time allows.

To check the reference index use:
php {binaryPath} referenceindex:update -c

To update the reference index use:
php {binaryPath} referenceindex:update