Feedback includes any information you get about yourself. In the broadest sense, it's how we learn about ourselves from our experiences and from other people—how we learn from life. It's your annual performance review, the firm's climate survey, the local critic's review of your restaurant. But feedback also includes the way your son’s eyes light up when he spots you in the audience and the way your friend surreptitiously slips off the sweater you knitted her the minute she thinks you’re out of view. It’s the steady renewal of services by a longtime client and the lecture you get from the cop on the side of the road. It’s what your bum knee is trying to tell you about your diminishing spryness, and the confusing mix of affection and disdain you get from your fifteen-year-old.
So feedback is not just what gets ranked; it's what gets thanked, commented on, and invited back or dropped. Feedback can be formal or informal, direct or implicit; it can be blunt or baroque, totally obvious or so subtle that you're not sure what it is.
In the boldfaced sentence, the authors are making the point that feedback need not always be explicit (what gets ranked). In fact, feedback is often implicit and open to interpretation (what gets thanked, commented on, and invited back or dropped).
Examples of explicit feedback, such as a performance review or a critic's review of your restaurant, are often such that they can be mutually compared and even ranked. For instance, based on the reviews of the food critic, one could conceivably rank the restaurants in the town, and your restaurant will earn a definite place, be it high or low, on that list. Similarly, performance reviews of different employees can often be directly compared, so that employee A can be said to have gotten a better review than employee B. When the authors talk about the kinds of feedback that can be ranked, that is what they mean.
However—and that's the point of the boldfaced sentence—not all feedback is like that. For example, suppose you notice your friend surreptitiously slipping off the sweater you knitted her the minute she thinks you’re out of view. That is also valid feedback: your friend clearly doesn't like the sweater. And this is true even though she presumably said that she did. This kind of feedback is implicit in her actions, where the friend is even trying to hide the actions themselves. The sweater is an example of what gets dropped.
Let's take another example of implicit feedback, this time of something that gets invited back (or not). Suppose some coworker, whom you barely know, decides to have a big birthday party. Many people get invited, including yourself. Next year, she has a much smaller party, but you still get invited. That's very positive feedback: clearly, she liked you being there. But since she actually didn't say anything to you, this feedback is implicit.