An unfinished sentence with an implied ending (i.e. the rest of the sentence would be something like "you wouldn't understand anyhow"). Usually when people say this, the implicit meaning is:

"I realize that you just asked a question, but the answer to that question is so obvious that even if I tell you the answer, you will probably be too stupid to understand it."

Sometimes it is said with a little more humorous undertones:

"I have known you for a while (or, You seem like a sharp fellow), so I'm sure that you don't realize how stupid that question is. Surely you really know the answer and just slipped up. If I smile while I say this, maybe you'll say something like 'heh heh how silly of me, of course', and then I won't have to publicly humiliate you."