The ventilation system works by extracting stale air from inside your house to remove moisture, odours, dust, bacteria and gases. This air is then replaced with allergen reduced fresh air from outside (unlike some systems that recycle air from a dust filled attic).
Heat Recovery Ventilation systems are really clever with their cross flow heat recovery core. When removing warm stale air from inside, this air passes through the cross flow core and preheats the incoming fresh clean air.
In summer it also works to cool your incoming fresh air.