Activated bleaching earth is a special kind of processed clay (usually made from bentonite) that acts like a super-absorbent sponge.
It is treated with acid to make it much more porous and effective, so it can pull out unwanted things from oils — especially color pigments, impurities, and other stuff that make oils look dark, smell bad, or go rancid quickly.
In simple terms:
It's a fine, powdery clay added to crude vegetable oils (like palm, soybean, or sunflower oil) during refining. It "cleans" the oil by grabbing and holding onto the bad stuff, making the final oil clearer, lighter in color, and more stable.
After doing its job, the clay is filtered out and discarded (now full of the impurities it removed).
Think of it as a natural filter for making cooking oils look nice and last longer.