Basically... it's giving only water without any nutrients towards the end of the life cycle of your plant. It's overall color will diminish and be much lighter a bit quicker than it would normally. The idea is that it will "flush" some of the excess nutes in the soil and roots area. It's believed this improves the overall quality of the finished buds you harvest soon after. Especially for smoothness.
Though there is a lot of debate over whether or not this process is effective at all, though most growers seem to swear by it. I don't think there is really a wrong answer.