Fission

Fission what we call a reaction when a large nucleus falls apart into several large pieces (as opposed to spitting out a small particle, like an alpha particle).

A common example of this is the fission of Uranium-235.

²³⁵U is "primed" by hitting it with a neutron making ²³⁶U.

²³⁶U immediately falls apart into ⁹²Kr and ¹⁴¹Ba and three neutrons.

Those three neutrons are important because if they then collide with other uranium atoms. Each of those will give out 3 neutrons...and so on...and so on.

This self-perpetuating process is called a chain reaction, and this is the driving force behind nuclear power and nuclear explosions.