Yes, that's exactly it. An oxidising agent in a fine mixture with a fuel. In the case of some explosives such as gunpowder they are different particles in the powder mixture; other explosives (nitroglycerine comes to mind, but I can't remember if I'm right) have one part of the molecule being an oxidising agent and the other the reducing agent. In the case of flour dust, the oxidising agent is the air; if it's not airborne, it's not explosive, hence why flour doesn't have 'CAUTION EXPLOSIVE' written on the bag.