I don't want to bore you with technical details, but the theory behind GPS technology is basically that you have the series of orbiting satellites, which transmit signals to a receiver. If you know where the satellites are, and if the satellites all have synchronized time clocks and the signals all arrive at your GPS receiver without being distorted or modified in any form, then your GPS receiver has a computer inside it that uses those signals to determine your location.
The problem is, that's the theory behind it, and all sorts of practical issues get in the way. I'm sure you've all tried to tune in radio stations that are too weak and are fuzzy because the radio station is too far away or your receiver is in a poor position. That's the sort of thing that can happen with a GPS receiver. It's receiving a radio wave, just like a radio receiver.
There are all sorts of other issues. The radio waves have to pass through the atmosphere to get to your GPS receiver, and as they pass through the atmosphere, they become distorted as well. All of these distortions—and there are other ones we could talk about—tend to create ambiguity concerning the location of the receiver. This is where you get into some more technical detail. Depending upon the complexity of your receiver, you can deal with these distortions to a greater or lesser extent.