An Analogy
I am obsessed with throwing the paper ball into the trash can from my working table. I could not score the ball into that trash can on my first attempt. After that, I can score the ball by decreasing or increasing the initial velocity if the ball lands behind or in front of the trash can.
Let me make the analogy complex. Instead of controlling only initial velocity, let me add another variable: the direction I throw the ball. In the language of control theory, the variables that we need to control are called control variables. I can score by observing where the ball lands and improving on the next attempt. Though it sounds easy, this requires great determination and practice to master. Here, I see the result of my initial effort and improve the control variable in the next attempt. This type of control is called negative feedback control.
The control system has different types of sensors that convert physical entities into equivalent voltage and current rather than biological sensors like eyes. We get the desired output by manipulating the output of such sensors.
Negative Feedback Control System
Let us apply 1 to the positive input and assume the negative input is 0 initially. Since the error is 1, the input at the gain block G is 1, and the output becomes as positive as possible. But what happens after output tends to 1? The negative input tends to 1, and the output of the summing block tends to 0.