Flammable and combustible liquids are a fire hazard. What does combustible mean?
Combustible simply means able to catch fire and burn easily.
When an object (solid, gas, or liquid) is combustible, it catches fire and will burn easily. For example, if you were to bring a flame near paper, it would burn very easily, while glass would not. Therefore, the paper is combustible and the glass is incombustible. One requirement for combustibility is the presence of Oxygen. If there is no oxygen surrounding the combustible object, it cannot catch on fire. This is why when you light a candle inside a jar and put a lid on it, the fire will extinguish very quickly.