Syukri,
Rail to rail supply voltage is the voltage between the + and - supply voltages of a device. When authors refer to "rail-to-rail output" capability, they mean that the devices is capable of providing outputs that are equal to the positive and negative supply (rail) voltages. When they refer to "rail-to-rail input" capability, they mean that the device is capable of operating with inputs that are equal to the positive and negative supply voltages.
Regards,
Kral