The salt is left behind. The steam is cooled back into relatively pure water. This system works well but it is very expensive. During the process, the salt can be separated from the water. The salt ions are attracted electrically to special plastic sheets.
Water molecules are too big to pass through and stay behind.
Algae and bacteria can dramatically reduce the amount of salt in sea water. Fill a small glass or paper cup with a mixture of salt and water. Place the lid upside down in full sunlight. Place the small glass with salt water in the center of the large lid. Place the jar carefully on the lid as shown.