Narrowband (narrow bandwidth) refers to a signal which occupies only a small amount of space on the radio spectrum -- the opposite of broadband or wideband.
This is entirely relative to what is being described; for example, an FM broadcast station takes up 150~200kHz on the FM band, whereas a TV station's audio is narrowband, taking up only 25kHz, and weatheradio broadcasts are even narrower than that. It is also very often used to describe radio antennas, called narrowband when they are designed specifically for one frequency or channel only instead of a wide range.
Narrowband can also be used with the audio spectrum to describe sounds which occupy a narrow range of frequencies. In telephony narrowband is usually considered to cover frequencies 300-3400Hz.