View Smith & Wesson's info directly here.
From the S&W Safety Notification:
"Smith & Wesson has identified a condition where the trigger bar pin could damage the lower trigger in certain M&P Shields in a way that may affect the functionality of the drop safety feature of the firearm, potentially allowing the pistol to discharge if it is dropped.
Any unintended discharge of a firearm has the potential to cause injury, and we ask that you STOP USING YOUR PISTOL IMMEDIATELY UNTIL IT HAS BEEN INSPECTED AND, IF THE CONDITION IS FOUND, REPAIRED."
Here is the link to a video from S&W showing how to evaluate your gun for this defect.