Before I explaining the "Merkle tree" or Merkle root" it is better to explaining what is "hash".Every block in a block chain has a hash. It's like a finger print. It identify the block and all it's content and it's always unique just as like fingerprint. Once a block is created it's hash is calculated. Changing something(data) inside the block will cause hash to change, in other other words we can say hashes are very useful when you want to detect changes in the block
So let's now move to the "Merkle tree" and Merkle root". Say we have 1600 transactions in a block( normally we can get avarage
[1] as around 1600 transaction per block, however it's depend on the size of the transactions). First we put each transaction through a
hash function[2] to get a transaction hash or transaction ID which is used itself to identify a transaction.
Then we put them all into pairs and perform a hash of that pair of hashes. We do put up them again and hash them again and keep doing this until all of the transactions in a block meets at a single hash. This single hash called as " Merkle root" and the cascade network called as " Markle tree".
That produced Markle root put in to a block header which is the part of a Bitcoin block that gets hash in process of mining. Having a Markle root in the block header, basically makes transactions temper proof. If you want to change value of it that would be reflected in the hash of that transaction or transaction ID. That change would cascade up the Markle tree to the Markle root, ultimately changing the value of Markle root. In thus in validating the block, everybody would be able to see that your transaction is not valid. It's not in a valid block since you're tempered with some how.
So if you're want to verify a perticular transaction, you only need to look at the branches that the transaction on the Markle tree. You don't need to look at the entire Markle tree just only the branches.
I think I have explained what i understood about "Markle tree" and "Markle root".
[1]-Link>>>
https://www.blockchain.com/charts/n-transactions-per-block[2]-Link>>>
https://en.m.wikipedia.org/wiki/Cryptographic_hash_function