In a nuclear fission reaction in a nuclear power plant, the radioactive element Uranium-235 is used in a chain reaction.
The fission of U-235 splits off two neutrons, which in turn strike two U-235 atoms.
Two neutrons are split from each of the two U-235 atoms. Each of these neutrons then go on to strike another U-235 atom. Each of those atoms are split releasing two neutrons, which go on and hit more Uranium atoms.
The chain reaction continues on and on, getting bigger and bigger with each split.
The things that slow down a chain reaction are the control rods. A control rod is made up of cadmium or boron, which absorb neutrons. If you insert the control rod between the uranium atoms, the amount of neutrons available to cause more splits is reduced.
In the second line up of dominos, the ruler served as a control rod. Putting it between two dominos breaks the chain reaction similar to what happens in a nuclear reactor.