Related: Bitcoin is [x] distributed [x] secure but NOT [] fast. It is a global clock that self-adjusts to tick roughly every 10 minutes, and that tick is used to extend an immutable ledger by adding verified transactions to it. The tick-time is chosen to allow for global propagation to happen. Incidentally this global clock can be used for other things than sending money, such as cryptographically tying something in the real world to a point in time.