What is difference between "fiber duplex" and "fiber simplex"?

Not sure of your context here, but in general terms, simplex (sometimes known as half-duplex) means only one device communicating at any moment and duplex (sometimes known as full-duplex) means both devices communicating. So if you have two devices connected together using 100mbit Ethernet in simplex mode, then the maximum theoretical throughput between the two is 100mbit while in duplex mode it's 200mbit.