A coin mixer is a smart contract that people use to make it difficult for 3rd parties to track their financial activities on the blockchain. It helps people defend their rights to financial privacy and does so by allowing them to break the link from a sending address to a receiving address.
To use a mixer you deposit your crypto into the mixer smart contract. When this happens your crypto is mixed in with everyone else’s crypto deposits and you get a “special receipt” file. You will then use this “special reciept” file to withdraw your deposited crypto. Note that you should wait a few days to make sure that it’s more difficult to track your transactions. The longer you wait the harder it is to track your transaction because it will get mixed up with more deposits from other users.
An example of a mixer is Tornado Cash, it allows users to obfuscate their digital trail on the Ethereum blockchain, check out the video below on how to use it.