The fire triangle is a diagram/concept which shows the relations of the elements which fire requires to burn. The three things necessary for a fire are: Fuel, Oxygen, and Heat. If one of these elements is lacking there will be no fire.
A nifty diagram of the fire triangle can be seen here:

or if you have a problem with URLs in WUs take a piece of paper (or use Paint or similar programs if you want to save paper), draw an equilateral triangle on it, then write the following words on the sides: 'FUEL,' 'OXYGEN,' and 'HEAT' with one word per side.

This should not be considered a cut-n-paste node because this is something which was introduced to me as a topic in my prescribed burn training. AFAIK, most people who don't light or put out fires don't know there is a named concept for the above common-sense relations. Having the fire triangle inculcated into every brain cell is important because when you're getting cooked like a rotisserie chicken you need to rely more on training and instinct than your panicked little brain.