Arming means that your system will detect events — in other words, motion detected from your Secure cameras or sensors — and record them in your timeline. If you have them enabled, you will also receive notifications.
There are two states you can use when arming your system. "Armed: Away" can be used while you are out of the house. "Armed: Home" can be used while you're at home, but still want to receive notifications or record events. Depending on your settings, you can receive notifications on your mobile device when motion is detected. From the Philips Hue app, you can then view video clips, look at the camera's live view, trigger light and sound alarms, or call a trusted contact or the authorities.
Disarming your security system will completely turn off all notifications and video recording.