A positive feedback loop is a self-strengthening spiral where a something is both a cause and an effect. Technology is a good example of a positive feedback loop. The more technology we have, the easier it becomes to come up with new technology. This and other "positive" positive feedback loops like knowledge, information and wealth have made our world very interesting, but could also lead to a disastrous positive feedback loop involving greenhouse gasses.
The planet is currently re-absorbing about a third of the CO2 we put into the atmosphere. The problem is that, once we cross a 2-3 degrees threshold, the planet's ability to absorb CO2 starts declining and eventually reverses, thereby joining us in pouring tons of greenhouse gasses into the atmosphere.
This threshold is called a "tipping point" as described in the attached video. (For those who don't understand German, clicking the "CC" button at the bottom of the video helps a bit.)
For more info on the science behind climate tipping points, get hold of "With speed and violence" by Fred Pearce.