Definition
Air conditioners and heat pumps move heat between two coils. The evaporator coil (indoors, near the furnace or air handler) absorbs heat and humidity from your indoor air. The condenser coil (in the outdoor unit) releases that heat to the outside. Refrigerant cycles between them to carry the heat along.
Why this matters for your home
Dirty or damaged coils are a top cause of weak cooling and high bills. The outdoor condenser collects leaves, grass clippings, and cottonwood; the indoor evaporator can ice over when airflow or refrigerant is wrong. Keeping both clean is a core part of seasonal maintenance.