This is the process by which a criminal tries to convert money and other assets that are acquired from crimes (these include drug trafficking and terrorism) into legitimate funds that will not attract any attention from regulators or other authorities.
This usually involves concealing their origin via a series of “layering” transactions designed purely for that purpose. These are often on uncommercial terms as the original crime is likely to have been highly profitable so a criminal rarely minds if some of their illegitimate gains are given up in the process of cleaning up “dirty” money and/or assets. Money laundering, failing to report suspicious transactions and/or otherwise assisting a money launderer are all crimes.