In a nutshell, instead of storing a current state of the data in a database, you store the "event", i.e. the information about the data being changed. You can a

In a nutshell, instead of storing a current state of the data in a database, you store the "event", i.e. the information about the data being changed. You can also additionally save the current state, either at the time the event is saved or independently, but that simply acts like a cache when reading data; otherwise you would have to replay the whole events history to get to the latest state.