GPS in one picture
Satellites high above the Earth keep broadcasting the time. Your phone listens to at least four of them and works out where it is from how far away each one is.
Satellites broadcast the time
Each satellite carries an atomic clock and constantly sends out a signal saying who it is, where it is in its orbit, and exactly when the signal left.
Delay becomes distance
The signal travels at the speed of light. The phone compares the send time with the arrival time: the delay multiplied by the speed of light is its distance to that satellite, so it must be somewhere on a sphere around it.
Spheres that cross
With three satellites the phone sits where three such spheres meet. Only one of their crossing points lies on the Earth, so that is the position.
The fourth satellite fixes the clock
The phone's cheap clock is slightly off, and a millionth of a second means 300 metres of error. A fourth distance gives a fourth equation, so the phone solves for its clock error too and gets both the right place and the right time.
The phone only listens
All signals go one way, from the satellites down to the phone. The phone sends nothing back, so the satellites do not know it exists and any number of people can use GPS at once.