The air handler is generally referred to as the indoor unit of a split system. Its main purpose is to circulate the air throughout the home.
The air handler works in conjunction with an air conditioner to circulate the cool air in the summer. Or, when paired with a heat pump, it circulates cool air in the summer and warm air in the winter.
During the colder times of the year, an electric heat strip, or “back-up heat” is used to help keep the home comfortable.