Nokia Lumia 820 - What is Double Tap to wake on my Nokia Lumia phone?

Double Tap to wake feature gives you an alternative for using the unlock key to wake up your phone. You can enable it via Settingstouch and then check Double tap to wake up phone.

Nokia Lumia Amber update will bring you Double Tap to wake feature, currently it is not supported for Lumia 720, 720T, 625, 620, 521, 520 and 520T.