The unstable nucleus of Uranium-235 can breakup in a process called fission.
Fission can be initiated by bombarding a U-235 atom with a neutron.
The U-235 is split into smaller (often radioactive) atoms, releasing free neutrons as well as heat.
The freed neutrons can bombard other U-235 atoms in a chain reaction.
The rate of the chain reaction is regulated by control rods that absorb excess neutrons.
The heat is harnessed by the nuclear reactor for energy.