It depends on the phone itself. Some have partial Glance and others have full Glance. At a minimum, it tells the time, if you have any alarms and/or quiet hours set, vibration, and your volume settings (if your ringer or phone is on silent). It also shows a couple of notifications for email, missed phone calls, new texts, and apps that you can select to show the status, such as Skype or WhatsApp.
It shows all of that on a black screen, and you can see that without unlocking the phone itself. Full Glance is the same, but I believe you can add Weather and a background to it, if you wanted.