ofc, you can't even start talking about moderation and not mention cross-sections

(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.