When you consume sodium your body needs water to compensate. Essentially, the more sodium you consume the more water your body is going to hold. If someone consumes a lot of sodium one day, typically the next day they'll be a lot heavier on the scales because their body is retaining the water to compensate for the increased amount of sodium in their system. The weight they gained is commonly considered as 'water weight' because it's water being held in by the system. If that person where to consume less sodium throughout the next few days and continue to drink lots of water, that water weight would be flushed out rather quickly. Hopefully this answers your question.