Every transaction has a hash associated with it. In a block, all of the transaction hashes in the block are themselves hashed (sometimes several times – the exact process is complex), and the result is the Merkle root. In other words, the Merkle root is the hash of all the hashes of all the transactions in the block . The Merkle root is included in the block header . It is therefore possible to securely verify that a transaction has been accepted by the network (and get the number of confirmations) by downloading just the tiny block headers and Merkle tree – downloading the entire blockchain is unnecessary. This feature is currently not used in Bitcoin, but it may in the future.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc