As far as I can tell, it's a standard which just means that the building needs very little energy for heating or cooling. It mostly comes down to making the building very insulated (as far as I can tell--not saying that's the complete story, but that's a big factor).
They are designed to reduce energy requirements (especially heating and cooling) by at least 75% to 90% compared to regular homes and ideally by 100% by capturing and holding heat in the winter and blocking heat in the summer. They tend to have thick walls, triple paned windows, and be air tight.
Passive house atleast here means a house that generates more energy then it consumes. E.g very well insulated house with solar power, ventilation that gives back energy etc.
