A feedback loop happens in a system when you are able to use an output of that system to influence the inputs, which in turn influences the outputs and repeats the cycle
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In engineering, feedback loops are useful to generate steady-state outputs. We had an entire subject dedicated to feedback loop controllers back in college. I enjoyed the topic so much I took the subject twice! An example of the use of feedback loops would be a thermostat that has a sensor to detect the current temperature. Once it senses the current temperature, it compares that with the target temperature and uses the difference in values to adjust the thermostat input. In this way, the temperature eventually reaches the target value
Humans are also machines subject to feedback loops. We enjoy receiving positive feedback. The more feedback we receive about how well we are performing, the happier we are and the more we are able to perform well. This is part of why we enjoy getting paid a salary regularly -- some part of our brain treats money as a way to "keep score" and as feedback that hey, you're not doing so bad, here's some cash to keep you going!
Feedback can be negative too, but the effect can vary based on the person's attitude. If you accept feedback and criticism of your work and use it to learn and move forward, then you can use even negative feedback as positive reinforcement. If you refuse to acknowledge legitimate negative feedback and instead take it personally or criticize the one giving feedback, then you are failing to learn from the feedback and it either becomes useless or in the worst case can even reinforce negative behavior
Quicker feedback loops allow humans to iterate faster and determine which behavior is best to achieve his goals. My favorite example of feedback loops is in games. Computer and video games are very good at providing near-instant feedback. Run into an enemy? Okay that means you take damage, so that's not good. Finish a task? Ah, you gained some experience points, so it looks like that's good. And games will even tell you immediately how far you are from achieving your goal! How many experience points until I unlock a new ability? How much more damage can I take before I die?
People enjoy this sort of instant feedback loops, so more and more systems are using some form of "gamification" to improve adoption and usage. Websites like stackoverflow grant you points for answering questions. Fitness trackers give you immediate feedback on how many calories you've lost. And so it goes
The downside of this instant feedback culture is that it becomes more difficult for us to appreciate things that have long feedback loops. And they are already difficult in the first place, since the feedback takes a long time to get back to you!
My favorite example of this is dieting. Dieting is hard mainly because you suffer regularly (due to having to eat food you don't like) but the effect isn't measurable immediately. It takes a while to notice any changes, so it takes more discipline to maintain the diet in the absence of that feedback.
This is why people have systems like calorie counting -- it provides an intermediate feedback to substitute for the feedback you don't have yet on how much weight you've lost or gained. But it's still an abstract concept and doesn't translate too easily into weight gained or lost. And also it can be difficult to track precisely since not everything you eat will have a calorie count available. I reckon that once we have an app where you just take pictures of the food you just ate it tells you how many pounds you gained or lost, a lot of people will be better at keeping to their diets
I like to look at the activities I pursue regularly and see if there is anything I can measure to help provide feedback loops for self-improvement. This is why I enjoy daily habits that track streaks -- it's simple, instant feedback that tells me I accomplished something.
Of course, humans are not machines that need to have perfect systems and be slaves to their feedback loops. So it's also okay to break your daily habits every so often or to cheat on your diet once a week or such, it is one of the privileges of being human. And breaking your patterns also provides some feedback which you can use later