OpenCL is a library that increase the performance of image processing software such as Darktable.
OpenCL is normally enabled by default, but not always!
To check it out the simplest is... to display it.
Go to the Global Preferences tab.
Then Darkroom > Image information line - and add
OpenCL: $(OPENCL_ACTIVATED)
To make your life easier, here's the line I use: $(FILE_NAME).$(FILE_EXTENSION) - $(MODEL) - $(EXIF_ISO) ISO - $(EXIF_EXPOSURE) - $/$(EXIF_APERTURE) - $(EXIF_FOCAL_LENGTH) mm - $(STARS) $ (LABELS_COLORICONS) - OpenCL: $(OPENCL_ACTIVATED)
Then just open a photo in the darkroom module and check if the information line under the photo says: OpenCL: Yes
If the answer is no, you can normally activate it in Global Preferences > cpu/gpu/memory > activate OpenCLsupport
But sometimes these fields are grayed out! don't panic, read this post