IAX2 multiplexing and trunking support.... didn't understand

Hi people!
I have red an article at wikipedia:

about the IAX2 protocol where is described that

IAX2 supports trunking, multiplexing channels over a single link. When trunking, data from multiple calls are merged into a single stream of packets between two endpoints, reducing the IP overhead without creating additional latency. This is advantageous in VoIP transmissions, in which IP headers use a large percentage of bandwidth.[/quote]

could you please explain me the definition of multiplexing in combination of IAX2?! I really didn’t get it at all.

For any explaination, I would kindly thank you.