Why Frequency division multiplexing splits the available bandwidth into several non-overlapping frequency slots, and each slot carries its own independent information stream simultaneously. All the channels are on the air at the same time, separated in frequency rather than in time. This is the basic idea behind broadcast band allocation, subcarriers in an FM stereo signal, and the many carriers used in OFDM.
Watch out The choice about merging streams into a digital combiner that pulse position modulates the transmitter describes time division multiplexing, where streams share one frequency by taking turns in time; the band-hopping choice describes frequency hopping spread spectrum.
FDM = separated in Frequency, all at once. TDM = separated in Time, taking turns.