Human nervous system is electrical in nature, and it can be damaged /killed by flows of current through the body. When human body becomes part of an electric circuit, that is electric shock. With a two-prong plug, if an electrical oven gets old and the insulation on the wires have been rubbed off over time, then the whole oven will be in elecrical contact with the wire. If a man trys to touch this oven, then the current will attempt to flow through human body into the ground. That is when electric shock happens. It is possible for a man to die because of this electric shock.
(The two-prong plug is not grounded. The grounded ones are called three-prong plugs.)