A heavy nucleus undergoing fission splits into two smaller nuclei and 2 or 3 neutrons with the release of energy. The neutrons released in the fission can go on to produce fission in three other Uranium-235 nuclei. These Uranium nuclei split to produce further neutrons which in turn trigger fission in further Uranium-235 nuclei and so on. This is referred to as a chain reaction.
Chain reactions are made use of in nuclear reactors and atomic bombs. In an atomic bomb Uranium is used above a critical size to get an uncontrolled chain reaction by ensuring all the available fissile material undergoes fission in the minimum time possible. In nuclear reactors the concentration of Uranium-235 is much less and the chain reaction is controlled in order to reduce it and more importantly stop it if required.