Hello Pravash,
FXS stands for Foreign eXchange Station. Typically, that's a port in the router which you would use to attach a telephone to it. This could also be attached to the external (telco) line of the PBX.
FXO stands for Foreign eXchange Office. Typically, you would attach the line from a telco here. Thus, that's the port you would need to attach to the PSTN.
Regards,
Dobromir