Refer to figure.
GNSS (Global Navigation Satellite Systems) are navigational systems that employ the use of many satellites orbiting the Earth (24 is a technical minimum). Each satellite transmits a constant coded electromagnetic message, called the PRN (Pseudo-random noise) as well as a navigation message which contains extra data useful to our receiver.

The PRN code contains immensely precise timing data from when it was transmitted by the satellite. This timing data allows us to measure the time between when a message was sent and when it arrives at the receiver. This can then be turned into a distance measurement. This distance means that we can draw a 3D sphere around the satellite, called a range sphere, in the knowledge that the receiver is somewhere on that sphere edge.

With multiple satellites in the sky above our receiver, this process can happen multiple times, every time giving us anther range sphere that narrows down a receiver's location further. For a 3 dimensional position fix to be made, we need 4 range spheres, and therefore 4 usable satellites in view. This is the basic principle of operation of all GNSS systems.

Not only this, but more modern GNSS receivers can also pick up very slight changes in signal frequency that occur due to the doppler effect.

The doppler effect is where waves (electromagnetic in this case) have a slightly higher detected frequency when the transmitter and receiver are coming together, but a lower frequency when moving apart (see the above annex, it occurs for EM waves and sound waves). As we know the exact frequency that the signal should be (L1 is 1575.42 MHz), and any expected movement of the satellites, our receiver can calculate its own ground speed using the combined doppler effect of all the satellites in view.