A blockchain is an incorruptible digital record of transactions. The information is grouped in 'blocks' of data that are then linked together in a chronological 'chain'. A block that has been added to the chain cannot be removed or changed.
Most blockchains are decentralized, which means stored, maintained and validated by lots of anonymous people all over the world - which makes them extremely secure. Every user has its own copy of the data and, whenever a new block is added to the blockchain, they spread the information of any new transaction to the entire network. This way, blockchain removes the need for an authority to verify the integrity of the data and it is not possible for anyone to alter the information in the chain.
This process creates a permanent archive of transactions that can be used to verify ownership and history of assets like NFTs.