What are the Merkle trees? What is its importance in blockchain?

Merkle tree is a fundamental part of blockchain technology. It is a mathematical data structure composed of hashes of different blocks of data, and which serves as a summary of all the transactions in a block. It also allows for efficient and secure verification of content in a large body of data. It also helps to verify the consistency and content of the data. Both Bitcoin and Ethereum use Merkle Trees structure. Merkle Tree is also known as Hash Tree.
The Merkle tree plays a vital role in blockchain technology. If someone needs to verify the existence of a specific transaction in a block, then there is no need to download the entire block to verify the transaction in a block. He can only download the chain of block headers. It allows downloading a collection of a branch of the tree which contains this transaction is enough. We check the hashes which are relevant to your transactions. If these hashes check out is correct, then we know that this particular transaction exists in this block.