It's a term for a money pool that breaks the transaction trail in crypto
Blockchains transactions are public, which makes tracking individuals and the parties they conduct those transactions somewhat straightforward. Mixers take coins from various wallets, "mix" them through various methods, then transfer them to another wallets. This is meant to obscure the link between the original source of the coins and their ultimate destination.
Crypto coin laundries.
Maybe only legit people would be allowed to use it. Clean money in, anonymous clean money out.
Clean money in, less anonymous clean money out. They would have to charge for the service.
The number of people who would pay to have their money anonymized for purely legal reasons isn't zero, but it seems less likely to be a going concern.