A gun safety mechanism provides an extra margin of protection against accidentally misfiring. While you should always point a gun in a safe direction and keep your finger off the trigger until you intend to fire, a safety mechanism prevents the firing pin from striking the primer when someone attempts to pull the trigger. In most guns, the safety mechanism blocks the trigger, preventing it from moving when the safety is on.
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A visual check of your gun will let you know if the safety is on without attempting to pull the trigger. When the safety is off, you'll see a red indicator appear on the safety's button or slide mechanism. Some rifles and shotguns with a slide safety may also have the letter "F" appear when the gun may be fired and an "S" appear when the safety is on. Other manufacturers add a green indicator to let you know when the safety is engaged.