An ecological tipping point is the point at which a chain of events are triggered that flip an ecosystem from one state to another.
A powerful example is the melting of the Arctic ice sheet. The snow and ice is of course rather white, a colour that reflects the sun's heat very effectively.
As average annual temperatures rise, the ice cap slowly melts away, exposing more of the dark blue sea, which in turn absorbs more of the sun's energy. This speeds the process of melting such that even if we were to prevent further rises in global greenhouse gas emissions the ice cap would continue to melt.
The effect of these tipping points is extremely difficult to predict, particularly if considering interactions between them. However, their very existence suggests that we must act with urgency to prevent a runaway ecological disaster.