The standard example of a "system" is a house thermostat. It's a heater on-off switch connected to a themometer. Somebody decides that the house should be at 68 degrees -- the "goal" -- and sets thermostat to turn the heater on if the temperature falls below that, and off when the temperature goes above. The heater is only either off or on. You can't really crank it up; it's just on more if you make the goal 75, and less if you set the thermostat at 60.
Donella Meadows uses a bathtub as her system example. There's water going into the tub from the faucet, and leaving by the drain. (Let's say there's a leaky plug.) The bather decides the goal of the system is six inches of water and adjusts the flow from the faucet to keep it at that level. The flow down the drain is "negative feedback," and the flow from the faucet is the "positive feedback."
The bather also has a goal for water temperature, and has a hot water valve (positive feedback) and a cold water valve (negative feedback), and a water heater, with a thermostat, in the basement.
A tub containing more water will hold its temperature longer. It will drain at the same rate, but be useful longer than a tub with less water to begin with. The capacity for water and the amount of heat in the water are both examples of "material stocks."
Meadows also mentions other systems connected to the bather and plumbing. There's the well or municipal water utility, the hydrological cycle, the bather's checking account, and the economy. This is interesting to me as an analogy for the system that includes gun violence.
I really don't have an agenda for or against gun control. (Alright, I'm tepidly pro-gun.) But I have a sneaking suspicion that the system that includes mass shootings is complex enough that the place where society can intervene is remote from gun ownership.