Why OFDM splits a high-rate data stream across many closely spaced subcarriers, each carrying a slow, narrow-band piece of the data. The subcarrier spacing is chosen so each one's spectral nulls fall on its neighbors' center frequencies, which is what 'orthogonal' means, so they do not interfere with each other. Because each symbol lasts much longer at the reduced per-subcarrier rate, multipath echoes fall within a guard interval and intersymbol interference is avoided. This is the modulation behind Wi-Fi, DVB-T, LTE and ham modes like FreeDV 2020 and some HF data systems.
Watch out The Fourier transform answer is tempting because OFDM is implemented with the FFT/IFFT, but OFDM is a modulation scheme, not a bandwidth compression technique, and it generally occupies a wide channel. The narrow-band, slow-speed description fits modes like PSK31 or WSPR, the opposite of OFDM's wide, high-throughput character.
OFDM = Orthogonal, so subcarriers are spaced to avoid interference between symbols. Many slow carriers beat one fast one.