Right now. Time for an event to begin or a decision to be made. Implies that there is no more waiting, no hesitation. ‘Zero’ refers to immediately as opposed to ‘the eleventh hour’ or an earlier time before a decision or action must be made. Typically, used as a military reference for an even to begin.
“You wedding planned for next week so you better decide if you really want to live with that guy forever. It’s zero-hour.”