The lut3d module will be introduced in darktable 3.0 and is designed to apply a 3D LUT (LookUp Table) to an image.
3D LUT A 3D LUT is a tridimensional table which allows to transform any RGB value into another RGB value. The most common applications of LUTs are film simulation and color grading. But they can be used for any other technical transforms like LOG to REC.709, which are used in video edition for example.
This is a companion discussion topic for the original entry at https://www.darktable.org/2019/05/new-module-lut3d/