Climate control: What does the "heat" button do?

This feels like a stupid question so I apologize in advance if it's been asked / answered before but what does the "heat" button do? I've never seen that on a car, it's either A/C or no A/C and the car decides what temperature to set inside the car based off of what temperature you choose and the conditions.

Do I have to hit that button if I want to heat the cabin? Confused here, thanks!