A system includes feedback loops, some balancing and others reinforcing. As Donella Meadows so succinctly puts it . . . a feedback loop is formed when changes within the stock affect the flows into or out of that same stock.
This is easily understood by thinking about your thermostat in your house. That devise continuously monitors the temperature in your house. You have a set point for your furnace, say, 68. As the winter chill penetrates your house, the temperature falls from 70 (no action) to 68 (still no action) to 67 (the furnace clicks on). For a time, the house temperature continues to fall since the furnace has to switch on for few minutes before the fan engages and sends the warmed air through the duct system.
Now, the house temperature, say, 64 (furnace on) starts to climb to 66 (furnace on) and then to 68 (furnace off!). For a while heat continues to accumulate and the house warms past 68. And then it will fall again, and the furnace will light again, and the house temperature will rise, and the furnace will switch off again.
All the while, the thing changing has been the ambient heat in the house, rising and falling within a single day. The thermostat both measures (thermometer) the ambient heat and then activates the heating system to keep the home system within a comfortable range . . . hence, thermo-stat.
The current temperature becomes the input in this closed system, turning the furnace on and then off. In this case, the feedback is called negative, since the change in the LEVEL (decrease in temperature) leads to an opposite change in the RATE (increase in heating). This feedback is sometimes called balancing, and the reasoning is pretty obvious.
In a school, one monitors the student work (what students are actually doing and saying) within classrooms, the best indicator of the vitality of the school-wide system. Team observations of student work and teacher practice (state or level of the system) lead to changes (professional development needs, eg) that will adjust the very work and practices initially observed. The state of the system adjusts the flows to that system.