So the fire triangle consists of heat, oxygen, and fuel. You need all 3 for a fire. In a wildfire scenario, you remove the fuel (grass, trees, etc.) From the fire by creating a dirt barrier. Basically scrape away all the vegetation down to soil in a line around the fire. The fire burns up to the dirt line, runs out of fuel, and goes out. That's what a fireline is. Its a dirt line used to stop the spread of a fire.