multiplexing refeers to the transmission of different signals in the same channel simultaneously. There are many forms of multiplexing that can be used, including frequency division multiplexing(FDM) where individual signals are transmitted over adjacent , non overlapping frequency bands. they are transmitted in parallel and simultaneously in time. In time division multiplexing(TDM) each signal is allowd to use the whole bandwidth for a certain period of time.
Multiplexing is needed so that the available channel can be used efficiently and also to save costs. There is too much to know about multiplexing, here is just a short overview. One thing that one has to know is multiple access.