It means there are nerves that run from your fingers and run through "choke points" in your arm, on the way back to your brain. When the sun hits your trigimenial nerve, it can cause a sneeze in the same way in that they are stimulated and the electrochemical reaction continues up the nerve. The squishy spot in between your thumb and your forefinger is one cluster of nerves, your carpal tunnel is another, then another in the meat of your forearm and the last one in the inside of your shoulder.