The subtype relation between types is analogous to the subset relation between sets. If we have one type Temperature that contains all temperature values and another type Winter-Temperature that contains only negative temperature values, then we say that Temperature is a supertype of Winter-Temperature. This also implies that Winter-Temperature is a subtype of Temperature.
The supertype relation, on the other hand, is the inverse of the subtype relation. Since TempResistor is a subtype of Resistor, then Resistor is a supertype of TempResistor.