What exactly do the 1 and 0 on the switch stand for

Hello everyone, Ebyte here.
As long as there is a power switch on almost all electrical appliances, lamps and sockets, two symbols "I" and "O" are bound to appear. If you only look at the symbols, what do "I" and "O" mean? Can you tell which one is connected and which one is disconnected? click here: https://www.cdebyte.com/Modem-RemoteIOmodule-EthernetIO