The second law of thermodynamics states that natural processes tend to increase the total entropy of a system and its surroundings, meaning that heat flows naturally from a hotter object to a colder one, and the reverse is not spontaneous. This law also implies that not all heat can be converted into work in a cyclic process, and that spontaneous processes are irreversible.
One way to state the second law is that heat cannot spontaneously flow from a colder body to a hotter body without any external work being done. This is why refrigerators and heat pumps require work to transfer heat from a cold area to a warmer one.
The second law is often associated with the "arrow of time" because it suggests that processes tend to move towards a state of greater disorder, making the past and future distinct.
The Second Law also implies that heat will always flow from a hotter body to a colder body, and it's impossible for heat to spontaneously flow from a colder to a hotter body without external work being done.