If the fridge has an icemaker many times the timer is an intricate part of the icemaker. If not the timer is usually in the back bottom along with the motor, compressor, and other “stuff”.
The timer controls the defrost cycle depending on the fridge it will change the cycle every 1 – 2 hours.
During the defrost cycle the fridge shuts down for 10 – 15 minutes. The compressor and the circulation fan(s) do not run, and a heat tape under the freezer bottom cover heats up and melts the frost build up before it is allowed to become noticeable in the freezer compartment.
This is also why food freezer burns in your refrigerator freezer much faster than it does in your deep freeze.