Coinjoin allows people to combine their transactions with random other people to increase privacy.
CoinJoin is a method for mixing coins in a transaction. The idea is that several users wanting to send funds co-ordinate their intentions by collating the inputs to their transactions so that a "master transaction" can be built. Once that's done then they verify it contains what they need in terms of outputs and change and sign the transaction. Only when all the users have signed it then it can be relayed to the network for inclusion in a block.