Negative pressure can occur when there isn't enough ventilation in the home and an exhaust fan is operating. Essentially, it has the effect of drawing air from any external opening in a house, including gas appliance flues and chimneys. This creates a problem with the operation of an open-flued gas heater, and particularly a faulty one – because it means that dangerous gasses, such as carbon monoxide, can be drawn into living spaces via the gas appliance flue or chimney.
Energy Safe Victoria have more information on he impact and causes of negative pressure in your home, including this explainer video.