On this Wikipedia the language links are at the top boiled mussels the page across from the article title. A frog sitting on the handle of a saucepan, which is sitting on an electric hob, which is glowing red. The boiling frog is an apologue describing a frog being slowly boiled alive. The premise is that if a frog is put suddenly into boiling water, it will jump out, but if the frog is put in tepid water which is then brought to a boil slowly, it will not perceive the danger and will be cooked to death.
If you drop a frog in a pot of boiling water, it will of course frantically try to clamber out. But if you place it gently in a pot of tepid water and turn the heat on low, it will float there quite placidly. The boiling frog story is generally offered as a metaphor cautioning people to be aware of even gradual change lest they suffer eventual undesirable consequences. It may be invoked in support of a slippery slope argument as a caution against creeping normality. In the 1996 novel The Story of B, environmentalist author Daniel Quinn spends a chapter on the metaphor of the boiling frog, using it to describe human history, population growth and food surplus.