How do nuclear reactors utilize chain reactions?

The energy released during the fission of, say, uranium, is used to heat up, vaporize, and use the resulting high pressure steam to move a turbine that is connected to a dynamo in order to produce electricity during a CONTROLLED chain reaction in a reactor.

Inserting special rods (made of boron, for example) into the uranium fissionable material (where the chain reaction is occurring) to absorb neutrons—the chain reaction's bullets that, when striking other uranium atoms, propagate the chain reaction—and slow down the reaction controlling it allows one to control the chain reaction, which is the breaking of uranium into two lighter elements with the release of energy and neutrons.

By maintaining a regulated fission process, nuclear reactors start and manage chain reactions while releasing energy for the production of electricity.