Blockchain is a public ledger of transactions which is shared between all participants within that blockchain's network. In theory. Blockchain is unbackable because its distributed and shared nature makes it effectively impossible for one actor to alter the ledger invisibly. Every computer in the Blockchain network is independently verifying the records.
Blockchains use a shared database, which utilizes a Peer-to-Peer protocol set up to allow the database to be updated whenever any user of that Blockchain updates the database. Rather than each individual entry being its own update, a large group of transactions are bundled together into a "block" of updates. Each block contains a cryptographic hash of itself, as well as a hash of the previous block.
Due to how the hashes are made, it's impossible to falsify an entry within a block without also changing the hash of the previous block. Therefore, if the hashes contained in a current block match the previous block's hash, it's considered valid and added to the public ledger. In this way, the block-by-block-by-block verification method creates a chain of validation, giving the protocol its name.