GPS stands for Global Positioning System, and it is a bunch of satellites orbiting the Earth in a network of sorts. This system was designed for military use and navigation, but now anyone is able to access these military-grade satellites throught the technology in their pockets! Because there are so many satellites in space, you are never out of range for at least 3 of them. This makes it easier to pinpoint the location of whoever pings off of the GPS signals.
GPS location is possible through a process called trilateration. Just as a venn diagram with three circles meets in the middle, trilateration uses three satellites to meet in the middle to see where you are. The GPS receiver uses overlapping spheres, like a venn diagram, to do this.
Scientists used to track satellites by checking the shifts in its radio signals, using a process called the "Doppler Effect." The US Navy did experiments in the mid 1960s tracking US submarines carrying nuclear missiles. There were 6 satellites orbiting the poles and the submarines were able to use the changes in Doppler waves and find the sub's location in minutes. This was around the Sputnik era, so things weren't as sophisticated as they are today.
Beginning in the 1970s, the Department of Defense (DoD) decided that the system the Navy scientists made was good, but they knew it could be better. They wanted a more permanent and stable system to use for their Navigation System with Timing and Ranging (NAVSTAR). The NAVSTAR satellite got up and running in 1978 and the entire 24 satellite system was fully operational by 1993.
This is the NAVSTAR GPS. This is where GPS really gained some traction