"Slow" Neutrons (around 2,200 meters /second) are easily captured by Uranium 235 when fission occurs and energy is released. "Fast" Neutrons (significantly more speed and energy than slow neutrons can cause fission and are used in smaller reactors (say one meter square or less) which need no moderator such as water or graphite, but need liquid metal first stage coolants.
There is "Resonance"neutron capture peak in the absorption cross section of U238 that prevents those neutrons from causing fission but leads to the production of Plutonium 239. Moderators help to reduce the capture of neutrons just above thermal energy at just over one electron volt energy. They pass through this energy in the moderator material.
U-235 is a fertile nucleus, it fissions efficiently on absorption of a neutron. Slow neutrons spend more time close to U-235 while passing by, than fast neutrons. So slow neutrons are absorbed by U-235 more efficiently than fast ones. That is why slow or thermal neutrons produce fission in U-235 much efficiently.