the salt stays on the area where the salt water was being stored. The way a system like this works is that the salt water stays in a bucket of some sort and when the water evaporates it is turned into its gas form and rises to the top, it then hits the clear roof area and sticks back into its fluid form as fresh water (since obviously salt cant perform the same trick the water just did). Once it's stuck to the top as droplets of condensation, those droplets follow gravity and roll down the sides of that roof. They roll all the way down the sides into reservoirs that were put into the raft, which can be tapped for drinking.

see here

*edit this is a great survival technique you should know. You can do the same thing by hanging clear plastic above collected bad water, and putting a cup in the middle of that collection. Put a rock or something right above the cup on top of the plastic making the low point of the plastic directly above the cup and it'll cause the droplets to go into it.