During defrost, a significant amount of water is produced. If this is not properly removed in freezing conditions, it can refreeze under the outdoor unit.
Base pan
The base collects the water that melts from the outdoor heat exchanger.
Base pan heater
In models for cold climates, there is an electric heating element in the base to limit ice formation.
Drainage
Drainage holes must remain clear and the unit must be installed high enough from the ground.
Risk of ice accumulation
A large mass of ice can reach the fan or the heat exchanger and cause a mechanical problem.