It's a signal from the brain. Telling us we're tired. Also, if you look at someone while they're yawning, you'll yawn too. It's just a message from your brain telling you that you're tired. Hope that was helpful.
Wow, that's a really really great question!! And, as I'm finding out by trying to research it online, nobody has a definitive answer yet! There are STILL studies being done about the mechanisms of, causes for, and behavioral effects of yawning! Yawns are complicated. There are SO many theories, I can't even list them all here. But here's what seems to be a scholarly summary in Wikipedia. The references listed are certainly valid and accurately quoted: http://en.m.wikipedia.org/wiki/Yawn#section_3 Yawns are contagious (studies on that, with both animals and human are mentioned too). -- I have already yawned NINE times while looking this up! .... Make that TEN. :O