feedback control
Feedback control is the trick of measuring what a system is actually doing, comparing it to what you wanted, and using the difference to correct itself — over and over, hundreds of times a second. Picture steering a car: you don't crank the wheel a precise number of degrees and hope; you watch the lane, feel the drift, and nudge. The 'error' (where you are minus where you want to be) is the steering signal. Take away your eyes (the feedback) and you'd be in the ditch within seconds.
The power of feedback is that it makes a system robust to things you never modelled — a gust of wind, a worn motor, a heavier load. As long as the loop keeps watching the error and pushing back, the output tracks the target even when the plant's behaviour drifts. This same loop runs your home thermostat, a drone's attitude, a hard drive's read head, and the cruise control on a highway. The price of that power is the risk of instability: push back too hard or too late and the system can oscillate or run away, which is why most of control theory is about loops that correct firmly yet stay calm.
The error signal is the heartbeat of every feedback loop.
The opposite is open-loop control: act on a fixed plan and never check the result (like a microwave running exactly 60 seconds regardless of whether the food is hot). Open-loop is simpler and stable by construction, but blind to disturbances and errors in your model.