In simple terms, a blockchain is a digital ledger. It's a public record of bitcoin transactions arranged in chronological order. It is a permissionless, distributed database based on the bitcoin protocol that maintains a continuously growing list of transactional data records. It is distributed so that each participant has the copy of the whole blockchain. The blockchain is shared between all bitcoin users.

It is used to verify the permanence of bitcoin transactions and to prevent double spending. It is secure and immutable, and it's also hardened against tampering and revision, even by operators of the data store's nodes.

Each blockchain record is enforced cryptographically, and hosts run machines working as data store nodes. A blockchain is made up of blocks. A block is a record in the blockchain that contains and confirms many waiting transactions, as shown at https://www.blockchain.com/explorer:

Roughly every ten minutes, a new block containing transactions is appended to the blockchain through mining. It is a file called the bitcoin blockchain sitting on thousands of computers across the world, perhaps even on your own PC at home. The file contains data about all bitcoin transactions—that is, the payment of bitcoins from one account to another—that have ever happened.

This is often called a ledger, and is similar to a bank ledger, which keeps a record of payments.