The ramjet engine is an engine which uses the speed of incoming air to boost its power and thrust to hypersonic speed. Now take it from the Air Force military ring man. I'm going to try to break this down to you although I don't really understand it very well myself.

The air coming in thru the inlet must be coming in at a very high speed before the ramjet will operate. These engines are not efficient at all at low speeds and they can't provide any thrust at all at very low speeds. Fuel is injected into the fast moving concentrated air coming into the jet. The rapidly expanding heated air produced in the flame holder is squeezed through the nozzle which produces massive thrust. This is the basic operation but if you imagine the effect of air being superheated at mach 3 and pushed thru the nozzle you can get an idea of the power which can be generated. Now let me ask a question? Imagine what would happen if you could produce this same effect on the air coming through the nozzle of an aircraft doing mach 6?