The leaves of the Merkle Tree has the hash of the transactions. How are transactions stored in the block? What is the purpose of Merkle Tree.
A Merkle tree is a hash function value which can be used to validate that every piece of information in a block can be authenticated, in this case, it validates that for each Tx and its hash value, it has not been tampered with in any way, shape or form.
Let's use an analogy: assume we have a file cabinet of files, each file in numerical order. Now, assume each file is sealed with a wax stamp, and each wax stamp has the date spelt in words. If we changed the date at all, the words make it so there's no longer a match (1==>7 is easily done, but not one==>seven!). So we're sure each file (Tx) is legit. Now assume the filing cabinet has the sum of the number of letters used in those words on the Txs' stamps. Any change that was made, say one==>seven has changed the sum of the letter, since seven is two letters longer than one. This is a simplistic analogy of how hashes can easily authenticate a piece of data.
Furthering the analogy, each block (filing cabinet) has a combination lock which references the block (filing cabinet )before that. So minor changes in even one single letter of a single Tx in a single filing cabinet (block) will invalidate whole system.
I recommend looking into the Wikipedia cryptographic hash entry to get a deeper understanding, or books like Mastering Bitcoin