What does it mean for a "baseband" signal to be a signal near zero frequency range?
Generally 'baseband' means the signal that you actually use, so an audio baseband would be 20 Hz to 20 kHz, and a telephone baseband would be 300 Hz to 3400 Hz. The bandwidth ratio, highest/lowest frequency, tends to be much larger than 1. Often a baseband signal will also contain DC, in which case a bandwidth ratio is meaningless.
If the baseband signal is transmitted by modulating it onto a carrier, then the resulting signal will generally be 'narrowband', it will have a bandwidth ratio very close to 1. An audio signal AM modulated onto 1 MHz carrier would have a frequency range from 980 kHz to 1020 kHz, practically a single frequency in ratio terms.