There are several possible ways to formulate the second law of thermodynamics.
One formulation refers to the maximum achievable efficiency of a heat engine.
As shown in the example of the Carnot process, this efficiency depends only on the temperature difference between two heat reservoirs and can never, in principle, reach the value 1. For that to happen, the lower of the two temperatures would have to reach 0 K, which is physically impossible.
Thus, the second law states that there is no way to convert heat into mechanical energy with 100% efficiency. The closer the two temperatures are to each other, the less efficient the conversion into mechanical energy becomes. For example, the maximum achievable efficiency of Otto engines is around 40%, although the exact value depends on several parameters.
An equivalent formulation of the second law states that heat always flows from the hotter body to the colder one, but not the other way around. A hypothetical, periodically operating machine that transports heat from a colder to a hotter reservoir without additional external energy would therefore contradict the second law and is called a perpetual motion machine of the second kind.
A modern example that illustrates this principle is the heat pump, which transports geothermal heat into houses for heating. The principle is closely related to that of a refrigerator. Since the ground is colder than the temperature inside a room, additional electrical energy must be supplied so that the heat can be transported against its natural direction of flow.