What does "it's only water gain/loss" mean with respect to talking about their diet?

People R 70% water: The make up of the human body is composed of a high percentage of water which permeates all of our tissues.Rapid changes of weight over a few day period or within the same day generally reflect shifts in water weight. Consumption of salty material may be followed by fluid retention & weight increase while a long day of working outside in the heat may drop you a few pounds.