Duplex on a switch or hub tells how two devices can talk to eachother. Half Duplex will only allow one device to communicate at a time. Half Duplex is like a walkie talkie, if someone is taking no one else can talk until the first person is finished.
Full duplex allows someone to send and recieve information at the same time.
A hub always works in half duplex as all devices share the same broadcast domain. Switches you can setup in half or full duplex as needed. Most if not all switches come setup in autoduplex, which will negotiate either full or half duplex with the device that is plugged into it.
Hope that helps,