As opposed to positive feedback.
A system with negative feedback will tend towards a certain stable state. The most famous example of this effect is the Watt steam governor, a device for regulating the flow of steam in a steam engine. The steam governor will decrease the flow if the machine is going too fast, and increase the flow if it is going too slow. This keeps the engine running at the desired speed.
So a system with negative feedback will tend towards a stable state, like a steam engine with a Watt steam governor, which was designed to keep steam engines from breaking from too much strain, from destabilising.
Example of a steam engine calibrated to run at 100 rotations per minute (10% negative feedback per time unit)
time machine 1 machine 2
0 150 50
1 145 55
2 140.5 59.5
3 136.45 63.55
t=inf. 100 100
Machine 1 is going too fast, so the RPM decrease over time until it reaches 100. Machine 2 is going too slow, so the RPM increase.
Negative feedback is generally used to regulate processes. Body temperature, as well as practically all bodily functions, is regulated by negative feedback.
Animal populations usually tend towards an optimum due to negative feedback. If there are too many animals of a species, there will not be enough food to feed them all, so more of them will die than usual. Deaths will outweigh the births, and population will decrease.
Conversely, if there are not enough animals, food will be so plentiful that almost all animals will survive. So the births will outweigh the deaths, and the population will increase.
Negative feedback keeps the world stable and alive.