http://blenderr6fles2eizk2hb5eoess3g2ltaudl3fqckev3kiu5zbrteuad.onion/faq
Anyone can see that Alice sent 1 Bitcoin to Bob, and they can also see the transaction history of both Alice's and Bob's Bitcoin addresses. Now imagine that Alice uses a Bitcoin mixer to send the 1 Bitcoin to Bob. The mixer takes in Bitcoin from Alice and many other people, and then sends out new Bitcoin to different addresses.