Trunks are specially designed links that can carry multiple and simultaneous signals. The signals can convey any type of communication data including voice and multimedia data. A trunk can consist of multiple bundled wires or fiber optics cables to maximize the available bandwidth or it may be a single high capacity line capable to carry multiple multiplexed signals.
Trunks solves major telephony problems. If trunks were not there, companies would have to buy separate telephone connection for every single extension that ran off its PBX. In VoIP, trunks are used to connect PBXs to internet. SIP trunks are commonly used trunks in VoIP telephony.