“When temperature difference exists in a body , there is energy transfer from high temperature region to low temperature region by conduction. And this heat transfer rate per unit area is directly proportional to the normal temperature gradient.”
To understand it clearly let us understand it with an example. Let’s take an iron strip which is heated at one end and the other end is free. Now, due to conduction the free end of the strip will get heated as there is heat difference between the heated end and the free end. Now the question comes how much heat will flow? So as the law says “this heat transfer rate per unit area is directly proportional to the normal temperature gradient” This means that the amount of heat which will flow from the heated end of the strip to the free end of the strip is directly proportional to the difference between the temperatures of the heated end and the free end. The more the difference, the more heat transfer would take place.