In Simple Words
Bitcoin transactions are not stored as one endless plain file. They are grouped into blocks.
Each new block is cryptographically linked to the previous one. That creates a chain of blocks — the blockchain.
Bitcoin nodes keep and verify this shared history according to the network’s rules.
If someone tries to alter older information, the cryptographic links change, so the alteration cannot happen unnoticed.
In simple words: Bitcoin’s blockchain is the shared, linked transaction history verified across the network.