Endotherms generate most of the heat they need internally. When it's hot outside, they decrease metabolic heat production to keep their body temperature constant. Because of this, the internal body temperature of an endotherm is more or less independent of the temperature of the environment.
Contrarily, Ectotherms' body temperature primarily depends on external heat sources. That is, ectotherm body temperature rises and falls along with the temperature of the surrounding environment. So if it is hot outside their body temperature rises as well.
