An uranium-235 atom absorbs a neutron, and fissions into two (fission fragments), releasing three new neutrons and a large amount of binding energy.
One of those neutrons is absorbed by an atom of uranium-238, and does not continue the reaction. Another neutron leaves the system without being absorbed. However, one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and more binding energy.
Photo Credit: Public Domain, https://commons.wikimedia.org/w/index.php?curid=522592
