A spectacle is something you can't believe you are seeing. Get on top of your desk at work in your underpants while playing the kazoo and you're making a spectacle of yourself.
The word spectacle comes from the Latin spectaculum meaning "public show," an apt translation because a spectacle, like a public show, is something worth watching. A ballet is a spectacle, or an elaborate production worth watching. Often the word is used to describe something that has a particularly exciting visual element to it — like an acrobatic display or a magic trick. It's something you have to see to really appreciate.