The USGA says “A player has “addressed the ball” when he has grounded his club immediately in front of or immediately behind the ball, whether or not he has taken his stance.”
This means you can take a stroke and strike the ball without ever actually addressing the ball. There are already a few times that you may have done this without ever knowing it. For example, while hitting the ball out of the bunker, or hitting the ball out of a lateral or water hazard.
This can come in handy in a situation when you think that the ball may move by grounding the club or by other circumstances. If you prepare to strike the ball by hovering your club behind the ball instead of grounding it you are not addressing the ball. If the ball moves you would not incur a one stroke penalty. Under rule 18-2b “If a player’s ball in play moves after he has addressed it (other than as a result of a stroke), the player is deemed to have moved the ball and incurs a penalty of one stroke.”