Rainwater is collected through the collar. It then trickles down the back of the coat where it is filtered through charcoal and "purified using a chemical process." The coat has a series of pockets that store the drinkable water, and a series of straws coming from those pockets so that you can sip when thirsty.
The Raincatch rain coat was designed by Hyeona Yang and Joshua Noble.
See the full article here.
(Thanks, Andrea!)