This idea was originally studied in the context of looking for a job. An employer is interested in hiring a new employee who is "skilled in learning." Of course, all prospective employees will claim to be "skilled at learning", but only they know if they really are. This is an information asymmetry. Skill in learning is malleable, and depends upon many factors, including diet, exercise and money.
Spence proposes, for example, that going to college can function as a credible signal of an ability to learn. Assuming that people who are skilled in learning can finish college more easily than people who are unskilled, then by finishing college the skilled people signal their skill to prospective employers. No matter how much or how little they may have learned in college, finishing functions as a signal of their capacity for learning. However, finishing college may merely function as a signal of their ability to pay for college, it may signal the willingness of individuals to adhere to orthodox views, or it may signal a willingness to comply with authority.