Objects that give off heat — or a feeling of warmth — include the sun, a radiator, and even your body. You can feel a marked difference in air temperature if you hold your hand near the heat of a candle flame, for example.
Heat can also mean the intensity of an emotion. In the heat of an argument, you need to watch what you say, because you may regret it later. The verb heat means to make something hot, like when you heat up last night's pizza to eat for lunch today. When things heat up, they become exciting or intense, the way thrilling chase scenes heat up your favorite action movie.