frequency-division multiplexing
Many radio stations broadcast into the same air at the same time, yet you hear only the one you tune to. They do not collide because each station is assigned its own slice of the frequency spectrum, and your receiver filters out everything except its slice. Frequency-division multiplexing applies the same trick to share one physical medium: give each signal its own band of frequencies, and they all travel together without interfering.
Mechanically, you start with several independent signals, modulate each onto a different carrier frequency so each occupies its own non-overlapping frequency band, add them together into one combined signal, and send that single mixture down the shared wire or through the air. At the far end, bandpass filters split the mixture back apart by frequency, recovering each original signal. To keep neighbours from bleeding into each other, small unused gaps called guard bands sit between the assigned bands. The total medium bandwidth is thus carved up into parallel lanes, each permanently owned by one signal for the whole time it is active.
FDM is everywhere the spectrum is divided: AM and FM radio, broadcast and cable TV channels, and the way a single fiber or coax carries many independent services at once. Its key trait is that each user gets a continuous, dedicated slice of frequency for the whole duration — efficient when everyone transmits steadily, but wasteful if a user's band sits idle while others are starved, because nobody else can borrow it. That weakness is exactly what time-division multiplexing addresses by sharing in time instead.
Your cable connection puts dozens of TV channels and your internet downstream all on one coax at once, each modulated into a 6 MHz frequency slot. Your modem simply tunes to the frequency band reserved for data and ignores the rest.
FDM gives each stream its own permanent frequency lane on the shared medium.
In FDM a user owns its band the whole time, even when silent, so idle bands waste capacity. That is the opposite trade-off from TDM, which hands the whole medium to each user briefly in turn.