Merkle Tree… what it is?  aka Hash Tree… It is a fundamental part of the Blockchain Technology.  It is a mathematical data structure composed of hashes of different blocks of data and which serve as a summary of all the transactions in a block.  It is an integral part of peer to peer protocols.  In most cases it has two child nodes under each node in a network.  Each transaction has a hash associated with it.  In a block all the transition cases are themselves hashed.  The Merkle Root is the Hash of all the hashes.