When you put the cells in a salt solution you are placing the cells into a hypertonic solution. That means the solute concentration outside the cell is higher than the solute concentration inside the cell. You should know from your own studies that water will move from areas of high water concentration to areas of low water concentration. A hight solute concentration means there is low water concentration. That means that the water will move from the onion cell and out into the hypertonic solution and the onion cell will shrivel up.
