"dieback" means the death of trees and plants. 

Feedback is when a system reacts to its current state - either to increase it or decrease it.

For example, you can have a concert with audio equipment. One type of equipment is the microphone (mike). The other type of equipment is the loud speaker. If you put the mike too close to the loud speaker, then the mike will hear the sound from the speaker and amplify it through the speaker, and amplifying that louderr noise, until the system is screeching and the audience is holding their ears and crying "turn it off!!!". This is an example of positive feedback. (a runaway process).

An example of negative feedback is the thermostat on your air conditioning (A/C). If the room gets too warm, the thermostat turns the A/C on. If the room gets too cool, the thermostat turns the A/C off. This keeps the temperature within a narrow range. 

This paragraph describes a system with positive feedback.The global climate gets warmer if there is more carbon dioxide in the air. Decaying vegetation (like trees) produces CO².  Live vegetation consumes CO².