Glance is a very low-power, low-visibility screen that displays the time and notifications whenever your phone is idle. You can actually set it various ways... always on, always on when charging, intermittent, and peek to further conserve battery usage... Essentially, it eliminates the need to bring the phone to life to check the time and to see if you have new mail, messages, tweets, etc.
Double-tap to wake was introduced simultaneously, but it is separate and part of the Navigation settings. With DTTW, Glance may not be as essential, since you can get the lock screen with a double-tap, but having the option of setting Glance to "always on" allows you to view the time without obviously doing so.