FXS/FXO are very simple….
FXS(Foriegn eXchange Station) and FXO (Foreign eXchnage Office) are loopstart analogue telephony interface, Wherein we connect analog telephones.
In FXS port we can directly connect analog stations. An FXS device which hosts the interface provides 50 volts supply to the line.
But in FXO port is basically used to connect the PBX. i,e FXO to a PBX and then to analog phones.