We have discussed fission and neutron capture. Now we are going to talk about how we can increase the likelihood that fission or neutron capture happening.

What happens depends on the energy (speed) of the neutrons. The neutron induced fission process is much more likely to happen if the neutrons have a lower energy than the ones that are generated during the fission process. So how are we going to slow them down?

This is done by placing the uranium in something which will reflect them and remove some of their energy by not absorb them. These substances are called moderators. Graphite and water are good moderators.
Wikipedia