An Acronym for Foreign eXchange Subscriber. The opposite of an FXO port and uses FXO signalling.
An FXS port provides a dialtone for an analog device (ie, phone), this is usually provided by your Telco or a PABX. A "Battery current" is also provided to keep memory powered in some models of phone, such as for last number redial.
Digium? make PCI cards such as the TDM400P that provide FXS ports that you can use with Asterisk. For larger installations you use a Channel Bank? which usually provides 24 FXS ports, and usually connect the Channel Bank? to your Asterisk box using a T1? or E1 connection.