Complex sentence: For transmission of near-zero-frequency-signals (e.g. human voice between 300Hz-3kHz) over a radio channel one has to upconvert the signal to a suitable frequency for transmission, the signal is converted from the baseband to the passband.
Split into simpler sentences: For transmission of near-zero-frequency-signals (e.g. human voice between 300Hz-3kHz) over a radio channel, one has to upconvert the signal to a suitable frequency for transmission. In other words, the signal is converted from the baseband to the passband.