A tipping point occurs in a system when it reaches a state such that a small additional change will eventually trigger a self-reinforcing feedback loop with disproportionately large and irreversible consequences.
A number of tipping points exist with regard to the earth's environment, some of which are dangerously close to being crossed, or have already been crossed, and the crossing of any one of these could eventually trigger the others. Perhaps the most dramatic example is global warming, which, if not stopped in time by a cessation of the massive (and largely unnecessary) burning of fossil fuels and other human activities which create greenhouse gases, will not only become unstoppable, but could even accelerate.
That is, the increase in greenhouse gases in the atmosphere has, or soon will, increase atmospheric temperatures so much that they will speed up the melting of the permafrost in the Arctic, thus causing the permafrost to increasingly release the massive amounts of greenhouse gases that it stores, thereby further contributing to global warming. Likewise, the rising atmospheric temperatures will also increase the temperature of the oceans, causing them to also release more of the vast amounts of carbon dioxide that they store. The global heating could additionally have various other effects that further strengthen this feedback cycle, such as altering global ocean currents and damaging the Amazon rainforest.
Another danger is with regard to the fact that tipping points, with their irreversible changes, may occur over a period of decades or centuries rather than just a few years. While abrupt on a geological scale, this might make them not particularly noticeable on a human scale, thus minimizing pressure to halt activities that are causing them.