I don't know about that specific model, but generally those locks are to lock the gun from unauthorized use. They generally have a "key", usually nothing more than an Allen wrench or some other type of tool to "lock/unlock" them. There will be a keyhole in the gun somewhere, you put the key in, turn it one way to lock, the other to unlock. It's not a "safety" in the conventional sense, but a lock for storage. I believe the M&P model has a glock type hinged trigger as it's "safety" but I've never actually even looked at one, that's just from what I've read.
Most people unlock them and throw the key away. If yours works, it's unlocked.