What's the Meaning of John 3:16?

Why is it all those sign flappers sitting behind the end zone at football games are always flashing their John 3:16 signs?  Quarterback Tim Tebow sports the quote just below his eyes for the world to see while TV networks reject commercials featuring references to it.  Can someone please help me understand what this is all about?

For those who are looking for answers easier to understand than some of Tebow's offensive play schemes, here's a video that may help.  Check it out...

http://religion.blogs.cnn.com/2012/01/10/explain-it-to-me-john-316/

For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. ~ John 3:16