Bitcoin don't have individual coins. Each transaction is like taking several pieces of gold and melting them together and splitting them up in new pieces. Mixi

Bitcoin don't have individual coins. Each transaction is like taking several pieces of gold and melting them together and splitting them up in new pieces. Mixing multiple transactions from multiple people do not reveal what came from where - you only know sending addresses, receiving addresses and amounts - but no links between which senders are tied to which recievers. Instead of having a bunch of individual historical lines for each "coin" and each person there's a big web of unknown addresses, with only a few known original senders and maybe a few known recievers. Coinjoin lets strangers securely pool together their coins and get the same amount of coins out to several new addresses of yours, except that now nobody knows which addresses belong together and which coins belongs to which sender.