Water weight refers to water in your cells and the spaces between cells that your body retains due to the presence of excess salt and sugar. When people begin to diet they tend to cut down on salt and sugar, which leads to a rapid loss of weight, but it's easy come, easy go. This isn't a loss of fat tissue, it's a loss of retained water, so it isn't what people usually mean by "losing weight" which to most means "burning fat."