A Merkle tree (hash tree) is a data structure that, when graphically displayed, resembles a tree. The principle of operation is that the data from the initial blocks is hashed, and then these hashes are summed up together at the next level of the tree. And this happens until they reach the root hash, i.e. until all the hashes grow into one.
This structure has gained popularity in blockchains, as it allows you to transfer large volumes of transactions, while leaving the possibility of verifying their authenticity with the Merkle proof: to do this, you only need to follow the “branch” with the desired block. There is no need to disclose all the data, the root hash and the verifiable data itself are enough.