Just Imagine that you have a glass which is full of hot water. When you put it in the environment where is low temperature than temperature of water then after some time water give it’s own heat to environment and become normal. That means that heat transfer to low temperature from high temperature. But why? Why heat transfer to low temperature from high temperature? Why heat does not transfer to high temperature from low temperature? Why water not become more hotter than it was before?
Heat transfer from high temperature to low temperature due to motions of molecules. Motion of molecules depends on temperature. If there is temperature is high then molecules have high kinetic energy that means molecules have high velocity and they move faster. But if there is low temperature then the condition is opposite. In this condition, molecules move slower than they move at high temperature because their kinetic energy will slow down and they move slower. So molecules will move from high temperature to low temperature due to difference in their motion and they carry heat to low temperature from high temperature.