In the scope of telephony and voice coding, Narrowband refers to audio spectrum frequencies of 300–3400 Hz.

This spectrum allows for speech to traverse and nothing else. This is the quality of voice achievable in landline PSTN phones (=regular telephony) and in most of the cellular networks.

In layman terms, narrowband provides crappy voice quality.

In WebRTC, G.711 offers only narrowband while Opus offers both narrowband and wideband capabilities.