You have 3 safeties on a modern S&W revolver. The top circle is showing the bolt as it blocks the hammer as the cylinder is open. The 2ND safety is on the bottom where the rebound block is. The hammer can't go forward if the trigger is not pulled. But years ago, this failed once and got a sailor killed, so they added another safety called a hammer block.
This is a little bar that floats in front of the hammer, shown in the bottom picture.
If your gun has an exposed hammer, play it safe, don't remove it.