Jokingly describing some one with narrow mind of the world, or have low understanding on everything, or some one with idiotic behaviors or mind. Single-digit refers to a number from 0 to 9.
Your friend: Hey did you check out Rob's new ride? It's off the hook.
You: Yeah I did. What I can't figure out is how he can afford a brand new Cadillac with only a single digit income. You know he's a bagger at Wallmart, he must have something going on on the side.