A warning label is a label attached to a product, or contained in a product's instruction manual, warning the user about risks associated with its use, and may include restrictions by the manufacturer or seller on certain uses.
Some of them are legal requirements (such as health warnings on tobacco products). Most of them are placed to limit civil liability in lawsuits against the item's manufacturer or seller (see product liability). That sometimes results in labels which for some people seem to state the obvious.