Boiling a Frog

There's a slang phrase in english for this. It is called boiling a frog, or boiling the frog. The idea is that if you have a frog and you put him in warm water, and then gradually,. very, very, very slowly, you increase the heat it. You start to heat the water, but not suddenly fast, very,very slowly. Is this true? I don't know if it's true. But the story is important. What is the meaning of this story about the frog and boiling it?