A blockchain is a ledger that thousands of machines keep their own copy of. New entries arrive in batches called blocks, and every block carries a fingerprint of the one before it, so changing old history would require redoing everything after it as well.
For a trader, the practical point is that settlement is public and permanent. You can verify a transfer yourself on a block-explorer instead of trusting a statement, but a mistaken send has no support desk to reverse it.
The tradeoff is speed and cost. A bank database handles tens of thousands of writes per second; a decentralised chain might handle 15 to a few thousand, and charges a gas-fee to ration that space. Most of crypto's engineering, from layer-2 to rollups, exists to work around that limit.
Related: block, consensus-mechanism, block-explorer, layer-1