It's always a pleasure to see you, and I hope I'll be able to repay your hospitality if you ever come to visit us in Cambridge.

repay 由前缀 re-（表示"再次"）和动词 pay（支付）组成，字面意思是"再次支付"，引申为"回报、偿还"的意思。在这里指用同样的热情款待来回报对方的热情好客。