An "Event" means something unusual happened around how the driver card or vehicle was being used, and the tachograph noticed it and recorded it automatically.
For example, a card inserted while the vehicle is already being driven, two cards being used in conflict with each other, driving without a valid card, or a brief loss of power to the tachograph.
You don't create these events yourself; the tachograph creates them the moment it detects one of these situations.
Events are different from Faults, which cover the tachograph equipment itself not working correctly, and from Infringements, which aren't recorded by the tachograph at all — they're worked out afterward from a driver's activity data, to check it against EU driving and rest time rules.
To see a driver's recorded events, see: