This refers to succession of transactions and buyers and sellers involved in a house move. Thus if we were to sell your house to a buyer, who in turn had sold their house to another buyer, who in turn had sold to a First Time Buyer, there would be four people in the chain.

All these links have to be continually checked during the transaction, otherwise should one link break at the end ie; the First Time Buyer pulls out because they cannot get a mortgage, then all of the sales will fall through.