Each USB port typically provides 500mA of power. If you are connecting more than one USB device to a USB port (e.g. by using a USB hub or USB keyboard with additional USB ports, etc.), then those devices will have to divide the 500mA between them. If the sum of the devices requires more than the 500mA provided by the port they are connected to, then you will receive a low power notice. To resolve this, connect less USB devices to the same port or use a powered USB hub (a USB hub that uses it's own power supply and provides power to it's USB ports).