What you learned, was it more like:

1. "You need to slow down neutrons so they can react"

or 2. "Here's the graphs of how the neutron absorption and scattering cross sections vary with neutron temperature for H-1, H-2, H-3, Be-9, C-12, O-16, Fe-54, Fe-56, Fe-57, U-233, U-235, U-238, Pu-239, …"

If it was the former, you didn't learn "nuclear engineering".

(which is a simplification in itself, but that's best left until 2nd-3rd year in uni)

But for general understanding, .. there is stuff that slows neutrons. some is more effective, some less. There is also activation. It is why tanks and ifvs were lined with polyethylene or similar on the inside back in cold war - it had lots of hydrogen. But for controlling a power plant that is not enough - why?

and then we answer why.