when you dissolve salt in water, then salt splits into ions and each can hold up specific number of water molecules, which means that this water can’t do water things. so on top of salt’s physical presence, there’s a fraction of water that can’t water, or salt water is less watery in a sense than fresh water. now it turns out, if you put a membrane between these two that allows water through but not salt, water goes to higher salinity on its own, because it’s less watery there. how hard it happens is quantified as osmotic pressure and it can be in tens of atmospheres. reverse electrodialysis is just a clever arrangement that avoids energy recovery from low volume of high pressure liquid, like how reverse of reverse osmosis would work