Multiplexing is basically a sharing trick in communication systems.
It means sending multiple signals or data streams over a single channel at the same time, without them stepping on each other—then separating them again at the other end.
Think of it like this
One highway ➝ many cars ➝ each car stays in its own lane ➝ everyone arrives safely.
Common types of multiplexing
Time Division Multiplexing (TDM)
Each signal gets a time slot.
Used in: digital telephony, GSM
Frequency Division Multiplexing (FDM)
Each signal uses a different frequency band.
Used in: radio, TV broadcasting
Wavelength Division Multiplexing (WDM)
Like FDM but with light wavelengths.
Used in: fiber-optic networks
Code Division Multiplexing (CDM / CDMA)
All signals share time and frequency but use unique codes.
Used in: some mobile networks
Why it matters 
Better use of bandwidth
Lower cost (one channel instead of many)
Higher data capacity
Backbone of modern networks (internet, mobile, fiber)
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