The earth wire
If a metal cased machine such as this cooker has a fault, so that the live wire touches the metal casing there is a problem. Without an earth it is likely that when the casing is touched a large current will flow through the cooker and the person and then into the ground. The user will get a powerful shock which may well be fatal. An earth wire is connected to every socket and metal cased electrical machine and then to to a copper spike hammered into the ground.
The earth is connected to the metal casing of the cooker and there is a fuse in the live wire usually near the switch.
If there is a fault in the wiring or the elements within the cooker, so that a live wire touches the cooker, the current will take the easiest path down the earth wire. Because it is an easy path a large current will flow melting the fuse and breaking the circuit and switching off the supply.
Many domestic machines are made from plastic. Because plastic does not conduct electricity an earth is not needed although a fuse is still used.
The video below has an explanation of the purpose and use of the earth wire from point 7m 55s for about 3 minutes