A blockchain is a distributed ledger of records (transactions) whose copies are stored on many computers in a network, with the records linked into a chain of blocks in which old data is practically impossible to change.
In plain words
Instead of one bank keeping the books, thousands of participants hold copies of the ledger. New records are added in blocks after being checked under the network’s rules (consensus). Each block contains a fingerprint (hash) of the previous one, so tampering with old data is visible to everyone. Blockchains can be public (Bitcoin, Ethereum, TRON) or private. A recorded transaction is normally irreversible, so a mistaken transfer is hard to get back.
Example. You send USDT to a friend’s address: the transaction lands in a block, the network confirms it, and anyone can see it in a block explorer by its hash, with no names but with the sender’s and recipient’s addresses.