What do you mean by the chain reaction in nuclear fission? How is it controlled?

When slow neutrons are bombarded on uranium-235 (${}_{92}^{235}\text{U}${92}^{235}\text{U}), each uranium nucleus splits into two nearly equal fragments (${56}^{144}\text{Ba}$ and $_{36}^{89}\text{Kr}$) with the release of three neutrons and tremendous amount of energy (nearly 190 MeV). These new neutrons can cause fission in the other uranium nuclei under the suitable conditions. Thus, a chain of fission of nuclei is formed which once started, continues till the entire uranium is consumed.

The chain reaction is controlled by absorbing some of the neutrons emitted in the fission process by means of the cadmium rods and then making them slow by the moderators (such as graphite, heavy water, etc). This way, the energy obtained in fission reaction can be controlled and utilised for the constructive purposes.