Final answer:
The question pertains to operant conditioning in psychology, discussing motivating operations that alter the effectiveness of reinforcers and discriminative stimuli that signal reinforcement availability.
Step-by-step explanation:
The concept being described is related to the principles of operant conditioning within the field of psychology. Operant conditioning is a type of learning in which the strength of a behavior is modified by reinforcement or punishment. In this context, the phrase 'alters the value of a reinforcer' refers to a motivating operation, which is an environmental event that can change the effectiveness of a reinforcer. On the other hand, the term 'signals a reinforcer is available' refers to a discriminative stimulus, which indicates that a particular response will be reinforced.