It is a fine dry powder made of a plant called Lycopodium. It is used as a fire effect since hundreds of years.
You need to sprinkle the powder into a flame, upon contact it will catch fire and make a massive fireball.
Let’s see what Lycopodium is exactly:
Lycopodium powder is a yellow-tan dust-like powder historically used as a flash powder. It is composed of the dry spores of clubmoss plants. When mixed with air, the spores are highly flammable because of their high fat content and their large surface area per unit of volume – a single spore’s diameter is about 33 micrometers, requiring about 30 laid side by side spanning a millimetre, or 750 to span an inch. Preferred sources are Lycopodium clavatum (wolf’s-foot clubmoss) and Diphasiastrum digitatum (common groundcedar) because these widespread and often locally abundant species are both prolific in their spore production and easy to collect.
Lycopodium has been used in fireworks and explosives, fingerprint powders, as a covering for pills, and as an ice cream stabilizer. Today, the principal use of the powder is to create flashes or flames that are large and impressive but relatively easy to manage safely in magic acts and for cinema and theatrical special effects.
-Use it carefully, don’t use it in wind.
-Don’t inhale during the explosion, because you may inhale the burning powder.