You assign your retainer a job (that you yourself must have) and you spend retainer credits, which are earnt from your grand company and probably other methods, to send them out on "ventures" to collect an item for you. They'll be out on their venture for a certain timeframe and when they return they will give you the item and earn themselves some retainer experience and level up themselves. You can gear your retainers too to increase the returns.

And yup it's going to be in 2.2