Tornado Cash is a smart contract system that allows you to send fixed denominations of Ethereum, and receive a cryptographic "note" that allows someone who knows the note to withdraw the same amount of Ethereum from the smart contract.

Since zero-knowledge cryptography is used to ensure the generated note cannot be linked to the depositing transaction, it can be used to send money to yourself or another person without revealing the identity of the sender. There are criminal and non-criminal reasons to do this.

Because it is a smart contract system, you do not have to trust a person or organization with the money. You do have to trust the smart contracts defining the system are correct. The smart contracts are publicly available to read and have been reviewed by many people, including software audit organizations.