This lists all the alerts, security, fire, air quality, etc. The detection time is when the sensor was triggered. The expiration time is when the event was actually fired, or alarm sounded. This is the detection time plus however many seconds you specified in the 'Delay before Alarm' field. If someone entered their PIN code on an Orbiter before the expiration time, then the 'reset before expiration' box will be checked, meaning the alarm was never actually sounded. These events are, therefore, normal occurrences. Every time you enter the home, triggering a sensor, but successfully turn the alarm off before the time expires and the alarm sounds, an entry will appear in this log with that box checked.
Reset time is the first time that a user entered the pin code on an Orbiter after an alert, and the User column indicates who entered the PIN code. This way, if there was a problem, you can see who was the first person to return home after that alert.