Too Long; Didn't Read
Tornado Cash mixer is a smart-contract that was placed ("deployed") in the Ethereum blockchain. Tornado is based on technology called zkSNARKs, a new cryptographic technology. It allows to build distributed networks, where computers perform large computations without the trust. Tornado team deployed a service for money laundering. It's not an easy tech, but everybody who understands it, knows that each working implementation can potentially become a breakthrough. For example - to defend your personal data from leaks from different services, to simplify misc verifying actions, that allows you to "prove that I have something, written in some document (ownership right, allowance to drive, absence in list of criminals, etc)"