Mystery Doug is a talking dog who answers questions from kids, he offers a short explanation In Why Do We Celebrate April Fool's Day?
Learn About April Fool's is a short explanation from WatchMoJo that was published ten years ago and is still good.
Perhaps the greatest April Fools' prank of all is the classic Spaghetti Tree hoax broadcasted by the BBC back in 1957. Here's a little interview with one of the producers of that hoax.