At its core, any money system is a ledger: a list of who owns what and who paid whom. Your bank balance is just a line in your bank's ledger. Whoever keeps the ledger has the power.
Thousands of copies instead of one
Bitcoin's ledger is not kept by one company. Tens of thousands of computers around the world, called nodes, each keep a complete copy. Anyone can run one on an ordinary computer.
Same rules, checked by everyone
Every node follows the same rules: no coins created out of thin air, nobody spends coins they do not own, nobody spends the same coin twice. When a new batch of payments arrives, every node checks it independently and rejects anything that breaks a rule.
Payments are grouped into blocks, about one every ten minutes, and each block is linked to the previous one. That chain of blocks, the blockchain, is the ledger.
Why nobody can cheat
To change the history, you would have to convince thousands of independent nodes to accept a version that breaks the rules. They will not, because each one checks for itself. There is no headquarters to pressure, no server to switch off.
▶ SEE IT IN THE MACHINEWatch each new block pass through the rings of the NODES, checked one by one before it is passed on.