Imagine an account book shared among thousands of people worldwide. No one controls it alone, everyone can verify it, and it's impossible to erase a line once written. That is blockchain.
No bank, no government, no company controls it. Thousands of computers share and verify every transaction simultaneously.
A recorded transaction is final. Impossible to modify or erase. What the blockchain has written stays written forever.
Anyone can verify any transaction on a blockchain explorer. No grey areas, no back-room dealings.