Multiplexing is the art of connecting spare network resources such as land-lines. Provide your networks with the necessary speed to support large numbers of customers and employees within your network.
A typical 200 Mbps land line connection will allow stable video or music streaming for about 20-40 clients, depending on the resolution. Multiplexing allows you to combine 2 or more 200 Mbps lines, supporting far more clients in a single network and providing a better experience.
With expanding internet infrastructure the next video transmission standard, already adopted by the entertainment industry, is 4K. This also means that videos and other visual data from popular web-sources such as Facebook or Youtube will become available in this format.
Given that most devices auto-select the video resolution according to available network resources, a modicum of 20 to 40 simultaneous users in your network could clog up a 200 Mbit connection.
Multiplexing is your way around this problem as it allows to distribute client requests uniformly over multiple connections in order to provide the best possible experience for the networks clients.
Sadly it is not magic, and you won't be able to achieve a 400Mbps download on 2 connected 200Mbps lines (except with P2P), but the total available throughput becomes 400Mbps. Allowing more bandwidth per device or more connected devices without performance deterioration.