Fat washing transfers flavors from an aromatic fat into a liquid, most often a spirit used in cocktails. The mixture is allowed to infuse, then chilled and filtered so the solidified fat can be removed. Brown butter, coconut oil and other fats give different results.
It is not the same as adding cream to a drink, nor does filtration guarantee microbiological safety. Follow a tested preparation and storage method, especially when using fats from cooked meats or dairy.
Reference: Further reading.