Blockchains are chains of blocks that contain information and are difficult to tamper with due to their unique structure. Each block has data, a unique hash, and the hash of the previous block, forming a secure chain. Tampering with one block would invalidate all subsequent blocks due to the unique hash connections. Hashes act like fingerprints, identifying blocks and detecting changes. To prevent tampering with blockchains, proof-of-work mechanisms slow down block creation, making it challenging to alter the chain. Overall, blockchains provide a secure and transparent method for storing data and transactions.