Republic, to put it into everyday, kitchen physics, it's like a tea kettle. But instead of a kettle on top of a stove eye, picture a steel tube full of water (the kettle), surrounded by an electric heating element (the eye). Turn on the element, and steam shoots forcefully out one end (the kettle's whistle).
Now, instead of an electric element, substitute layers of radioactive materials which will generate a LOT of heat, just like our power plant reactors, and instead of water sitting in the tube, substitute cold hydrogen flowing through. Make the nozzle right, and the hydrogen "steam" gives you a LOT of thrust.
Simple, no?