People often say heat rises, but that’s not entirely true. While hot air weighs less than cold air and will naturally rise, heat travels in all directions; from hot to cold. Heat flows between substances when there’s a difference in temperature. If you leave a 32°F ice cube sitting in a 69°F room, heat from the room will transfer into the ice causing it to melt. If both the ice cube and the room are both 32°F, no heat transfer will take place. Theoretically can extract heat from a substance until absolute zero, -459.67°F (−273.15°C) where all molecular motion would stop.
Heat is molecular motion. The faster molecules are moving, the more heat in a substance. Think about it like the more rapid the molecules move, the more friction created, which in turn causes heat.
Most of us would think a cup of ice water is cold, but there’s still heat (molecular motion) inside that cup of ice water, just not as much as a cup of boiling water. The boiling water will have a higher amount of molecular motion, and in turn, more heat.