Whenever an evaporator runs below 0 °C, moisture from the cabinet air freezes on its fins. A thin layer of frost does little harm, but as it thickens it blocks airflow and insulates the coil. Defrost clears it. The controller stops the compressor, sometimes stops the fans, removes the frost, waits for the water to drain, then restarts cooling.
There are three main methods: off-cycle defrost, which simply lets the coil warm with cabinet air; electric defrost, which uses heater elements; and hot-gas defrost, which sends warm refrigerant through the coil. Defrosts are usually scheduled at set intervals, and some controllers start them only when needed, based on run time or coil temperature.
Humidity drives frost. In monsoon months, or in a busy kitchen with constant door openings, a freezer may need more frequent defrosts. If frost keeps returning fast, check door gaskets and opening habits before changing settings. Manual-defrost chest freezers need periodic emptying and defrosting by hand when ice builds to a few millimetres.