An Inductor Coil is a block, usually metallic, that can be placed around the Rotor shaft and that, while engaged, will extract energy from it while slowing it down.
A particular coil may be more efficient at extracting energy from the spinning Rotor than another one, or may extract energy more rapidly then others.
Usually, a rare block or one that is 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 as Inductor Coils.
Other mods or mod packs can add their own Inductor Coils or modify/remove the default ones.
Only valid coils can be placed inside the Turbine, around its Rotor shaft. Invalid blocks will prevent the Turbine multiblock from forming.
Blocks that can be used as Inductor Coils are marked with a special tooltip that you can see if you want to spoil yourself, provided you have enabled the Advanced Tooltips feature of Minecraft (F3 + H).