FXS stands for foreign exchange subscriber. An FXS port is an interface that connects station devices such as your phones or PBX to a VoIP adapter.
FXS is an RJ11 port that connects internally to an analog office phone or fax machine. Think of the S as meaning a station or a cubicle. Any FXS port is going to connect to an analog DEVICE and the cable from the port to the device will never leave the building.
Now, on to the FXO ports…