Temperature is a property of every particle. Temperature can change material's behavior or make them transform into other materials.
Most materials conduct heat to other particles nearby. This is called heat conductivity. Some materials have higher heat conductivity, while some do not conduct heat at all.
Ambient Heat was added in version 50. Some saves may require ambient heat to be switched off, while some require it to be on to function properly. Ambient Heat means that the air itself also has it's own temperature.