The second law of thermodynamics states that heat engines can never be 100% efficient. Only *some* of the heat entering the engine from a high-temperature reservoir can be converted to useful work. The rest of the heat is expelled to a low-temperature reservoir.
Work Step by Step
For example, a car's engine receives heat (from a high-temperature internal combustion chamber where fossil fuels are burned), turns the crankshaft that drives the wheels, and exhausts some heat from the tailpipe (to the low-temperature environment). In other words, the second law tells us that every heat engine must expel some heat. No engine can convert all of the input heat to work. This is discussed on page 345.