A radiation moderator is a block or fluid that can be placed inside the Reactor to absorb part of the radiation emitted by the fuel and move heat away from the Fuel Rods.
A particular moderator could convert radiation to heat more or less efficiently than another one, or could absorb more radiation than others, or it could conduct heat way better than its companions.
Or it could be dirty cheap but extremely inefficient all around. Like air, for example: it works, but barely.
Usually, a rare block/fluid or one particularly expensive to craft has better stats than a common or cheap one.
Extreme Reactors register, by default, a score of vanilla and modded blocks/fluids as moderators.
Other mods or mod packs can add their own moderators or modify/remove the default ones.
Only valid moderators can be placed inside the Reactors. Invalid moderators will prevent the Reactor multiblock from forming.
Blocks and fluids (check the bucket) that can be used as moderators are marked with a special tooltip that you can see if you want to spoil yourself, and you have enabled the Advanced Tooltips feature of Minecraft (F3 + H).