WordPress developers use transients to speed things up. Transients allow them to reuse specific information, which makes themes & plugins to work faster. But sometimes, because of that, there might be some errors or content that doesn't get updated. Just like you empty cache, you may want to delete those transients to fix things quickly.
All transient-related database entries are deleted after clicking the button. That includes two DB entries per transient – one that holds the data, and the other that holds the expire timestamp. Any orphaned entries are removed as well. Expired and non-expired transients, too.
The tool runs one DB query to delete transients. The table name is pulled from the default WP
$wpdb->query("DELETE FROM $wpdb->options WHERE option_name LIKE '\_transient\_%' OR option_name LIKE '\_site\_transient\_%'");