The philosophy of multicarrier modulation is: a large number of subcarriers (L) are used in parallel, so that the symbol time for each goes from T-->LT. In other words, rather than sending a single signal with data rate R and bandwidth B, why not send L signals at the same time, each having bandwidth B/L and data rate R/L ? In this way if B/L is less carrier BW, each signal will undergo approximately flat fading, and the time dispersion for each signal will be negligible. As long as the number of subcarriers L is large enough, the condition B/L is less than carrier BW can be met.
This elegantidea is the basic principle of orthogonal frequency division multiplexing (OFDM).
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