Traffic Engineering (TE) is both a profession and a technical concept. As a profession, a classical TE is a person who manages automobile or public transportation traffic; for instance, TEs are people who time the stop lights and set up bus routes. However, a TE in the Internet realm is a quite a bit different.
A network Traffic Engineer is a person who provisions communications routes. For instance, a TE may assign specific timeslots across multiple SONET rings across the continent for a customer. Or, a TE may be one who designs Quality of Service QoS policies for network traffic: which brings us to the last definition.
Traffic Engineering is also the act of balancing the traffic loads on routers and links within a network by using parallel, alternative paths and a signalling protocol to manage the flow of data amongst these paths. For instance, BGP and IGP-like protocols (e.g. like OSPF) fill TE needs for a given network.