An old method of transmitting information (voice or data) through a system or network by establishing a persistent, dedicated connection ("circuit"). This connection consumes resources even when no information is being actively transmitted, which is inefficient. This type of connection also takes time to establish, which introduces noticeable delays when using data services.
The alternative is packet switching, which is far more efficient, and enables much faster data rates.
See: Packet Switching
For data moving over mobile networks, circuit switching was phased out with the introduction of 3G technology.
For voice, circuit switching was phased out with 4G LTE (VoLTE, specifically).