Look at the picture and try to figure out where is it... Have you ever been to the UK? Ok, it's in the UK... If it's a known place? Yes, very famous... Ok, it's London! Well... it's near. See those tall buildings on the top right? That's actually Canary Wharf. Where did a take the photo? Greenwich. Yes, it's near Greenwich's Royal Observatory where the longitude is 0º. It's also the reference for the GMT time zone. Or now often refer to as UTC, Coordinated Universal Time. You know it's 8am where you are, based on your distance to this place. GPS will tell you how far you are from this place to East or West on Earth. Still, most people (like me :) ) would just want to play on the beautiful grass. A reference is something you just need to know where it is to relate to it. It doesn't mean that you have to know how it looks like.