A cryptocurrency mixer, or tumbler, is a service designed to break the traceability of blockchain transactions. It takes coins from many users, pools them together, and pays out equivalent amounts from the mixed pool — so that following the money on a blockchain explorer no longer connects a sender cleanly to a receiver.
Mixers are used both for legitimate privacy and for laundering the proceeds of crime, which puts them at the centre of many financial investigations. For an analyst, a transaction entering a known mixer is a significant event: it marks the point where a clean trail may go cold, and it is itself a signal worth noting, since heavy mixer use often accompanies illicit activity.