the heat given off as a consequence of energy transformation is usually referred to as "unusable energy......"

Question

does this mean it can no longer be used? why?

Answer 1

It is not useful to the original process which created it. for example, 70% of the heat generated in a petrol engine is not used in powering the engine and is wasted. Only 30% does useful work such as pushing car.

Answer 2

It is referred to as "unusable" energy because all it is doing is increasing the entropy (disorder) of molecules in the surroundings. Once the heat has been distributed throughout the surroundings, it can no longer perform useful work, and so is unusable.