Play with fire

This short expression is used to express danger.
Literally, it means to use fire as a toy, which can, of course, be dangerous.
So, we may hear a worried mother say something such as, “Jack! Don’t play with fire!”

The expression can also be used figuratively to mean doing something dangerous or risky.
In this way, buying stocks in an unknown company is playing with fire.
Likewise, you would be playing with fire if you decided to eat 3-day-old fish!

この表現は、危険を表すことに用いられます。
文字通り、火で遊ぶということはもちろん、危険ということです。
なので、心配そうに母親が「ジャック！火遊びをしちゃだめよ！」と言っているのを聞こえることがあるかもしれません。