They say that if you put a frog into a pot of boiling water, it will leap out right away to escape the danger. But, if you put a frog in a kettle that is filled with water that is cool and pleasant, and then you gradually heat the kettle until it starts boiling, the frog will not become aware of the threat until it is too late. The frog's survival instincts are geared towards detecting sudden changes.
This is a story that is used to illustrate how people might get themselves into terrible trouble.
This parable is often used to illustrate how humans have to be careful to watch slowly changing trends in the environment, not just the sudden changes. Its a warning to keep us paying attention not just to obvious threats but to more slowly developing ones.