To know what's what means to be very experienced and knowledgeable in a shrewd way.
1. All the soldiers knew they could rely on their group Captain to get them out safely from the enemy's territory as he knew what's what.
2. The coach had trained his wards well as he knew what was what about the rival team's tactics.
3. Tehzeen could always be relied upon to get the best facilities for a convention as she knew exactly what's what about organizational aspects.