If you are playing with fire you are doing something which could be dangerous.
Climbing that mountain in this bad weather is playing with fire.
He was playing with fire when he swam across that river at night!
If you add fuel to the fire it means you make a bad situation even worse.
John's poor excuse for being late just added fuel to the fire. Jane was furious.
The verb 'to fire' can also mean to shoot. In military battles, soldiers were commanded to "Fire!" when they had to start shooting at the enemy. 'Enemy fire' is what they got in return.