A system comprised of multiple satellites in space orbiting the earth, transmitting precise radio timing signals, and receiver devices on earth that use the signals to determine their precise location on the planet.
There are many types of receiver devices, but nearly all modern phones include a GNSS technology.
There are multiple GNSSs, created by different governments and organizations around the world. Each system has its own separate fleet of satellites, although many intentionally use overlapping radio frequencies, making it easy for receiver devices to use multiple networks at the same time, which improves accuracy and speed.
GPS is the GNSS created by the United States.
GLONASS is the Russian system, Galileo is run by the European Union, BeiDou is the Chinese system, and India's is called NavIC. Japan also has a system called QZSS, which initially works by piggybacking on GPS, but is planned to eventually be a fully independent GNSS.