A transaction groups several statements, such as inserting one order together with its two line items. COMMIT makes every change permanent, while a failure or ROLLBACK removes the uncommitted changes, so the order is never left stored without its line items. This all-or-nothing guarantee is the atomicity part of ACID. The panel's t1, t2, and so on are not real timestamps — they are step numbers marking the order of events along a shared timeline. The later lessons connect the other ACID properties to concurrency and committed data.