The video explains how the touchpad is located on the eyeglass frames between your temple and ear. You can "wake up" Glass by tapping the touchpad. You can manually adjust the position of the homescreen image so that it "sits" above your line of sight.
Operating Glass with the touchpad is pretty simple. With your finger, swipe back and forth on the pad to scan individual cards that make up the Glass timeline. Tap the touchpad to select a card if you want to see more information. The cards to the left are things happening currently -- weather, incoming flights, and calendar events. To your right are things like messages, videos, and photos.
Swipe down on the touchpad if you want to go back to the beginning, and then once more if you want Glass to go back into standby mode.