If you say that someone is playing with fire, you mean that they are doing something dangerous that may result in great harm for them and cause many problems.
When fire was the exclusive property of the gods, it symbolized the divine power to make the world do what you wanted it to do. When you think about it, that's pretty major. So Prometheus decided to make man stand upright as the gods did and to give them fire. He was attempting to manipulate something very dangerous, the gods. it was like he was "playing with fire"
