That's the gist. It's not much, but if the outside unit is iced over, there's not going to be a lot of heat exchange into the working fluid, so you're not going to get heat into your conditioned environment.

So, a defrost system will include a temperature sensor, and a resistance heater, and often something to signal the inside unit to run the emergency/auxiliary heat until (unless) the outside unit can get up to a temperature where it will work.