The Inhibitor Mod is a Terrain Toolaugment, which prevents your Terrain Tool from deforming the surface you are using it on.
However, when collecting on a surface with resources on or in it, the resource will collect to a set depth, depending on whether or not you are using the Narrow, or Wide mod. This will, however, work on resources that require the use of a Drill, i.e. Lithium, or Titanite, when used with a drill mod, but the inhibitor mod must be in the top slot.
It also allows the dislodging of surface rocks, stalagmites and stalactites without damaging terrain.