An efficient means of routing and transferring data over a network by breaking it up into very small pieces (packets). Each packet is addressed to its destination, like pieces of mail in a postal system. A great many packets, from a large number of senders and recipients, can move through a data network simultaneously and efficiently.
Essentially all data (including voice) is now packet-switched. This was always how the internet worked, and is now the basis for mobile networks as well.
The alternative to packet switching was circuit switching, which was inefficient. Circuit-switched connections took time (up to several seconds) to set up, and used resources even when inactive.
See: Circuit Switching
Packet-switched data was introduced to mobile networks with 3G technology. This brought faster data speeds and "always-on" data.
Later, voice calls also moved to packet switching by essentially treating audio as just another type of data. This happened with the introduction of VoLTE technology during the 4G era.