Wikipedia: Port triggering is a configuration option on a NAT-enabled router that allows a host machine to dynamically and automatically forward a specific port back to itself. Port triggering opens an incoming port when the user's computer is using a specified outgoing port for specific traffic.
Port-Forwarding.net: This is an option of router. Basically, port triggering and port forwarding are the same. They all open a port on your router and when there is connection or data from internet, your router will forward the connection or data to your local computer that behind router. The difference between port triggering and port forwarding is, when you use port forwarding function of your router, your router will always open the port no matter you are using it or not and the internal address be forwarded is static, if you want to change the address that you want to forward, you must edit the port forwarding entry in your router. Comparing with port forwading, port triggering is a little more secure and it is dynamic. But the disadvantage of port triggering is unstable, it really depends on your rouer. Your rouer will close the port after some period of time if there is no data pass through. This will break your application down.