An explosive is a substance that can be made to explode by being hit or lit on fire. You might use an explosive to create a tunnel through a mountain — but you probably shouldn't use one to clear weeds from your yard.
Explosives contain highly reactive chemicals that are full of potential energy, which can be released with a bang. You can also use the word explosive as an adjective, to describe such substances or anything else that is dramatic, abrupt, or loud. You might say that your brother has an explosive temper sometimes, or that your snow shoveling business has seen explosive growth this winter. The Latin root is explodere, "drive out."