Rating 1.5V 50mA (MAX) what does it means?
1.5V is the required supply voltage. It should be like this +/- some tolerated deviance. the 50mA MAX is the maximum current the manufacturer of the mouse is promising will be drawn by it under any (functioning) conditions. These two specifications are necessary for choosing the right power supply for the device - it should be able to provide this maximum current with the specified voltage.