Feedback can be used to maintain a desirable steady state. This may be the water level, temperature, voltage, or even the rate at which these things change. There are two types of feedback.
Negative Feedback
If the temperature in an incubator falls too low for example, the heater is switched on and the temperature in the incubator goes up. If the temperature gets too high the heater is switched off. This type of feedback is called negative feedback.
Positive Feedback
Positive feedback is less common than negative feedback, and often destructive. It can lead to explosions, or contribute to global warming as illustrated below.
Sometimes though, positive feedback is desirable, especially for armies, who like destroying things. Positive feedback takes place in bombs, most dramatically in atom bombs.