Cross ventilation is when a building is set up to utilize the natural winds that blow across a site to assist with cooling the building.
Simply put the windows are so placed and the internal structure of the property (i.e. placement of doors and open spaces) set up as to allow the easy movement of air from one side of the property to the other when the windows and doors are open; so ventilating the property essentially for free.
This is often a key component of passive solar and thermal mass design. In that during a hot Summer night the windows and doors can be opened to let in the cooler night air, this in turn cools the thermal mass, which the following day helps assist in keeping the property cool.