It is significant that the contracts between the main contractor and the customer and between the key contractor and subcontractors are back-to-back; what is meant by that term?

The phrase ‘back to back’ implies that any contract terms applied to the main contractor are ‘flowed down’ to subcontractors. It is significant as, otherwise, the major contractor may determine themselves liable for things which they have entrusted to others, along with no legal redress for their subcontractor’s failings.