In the electrochemical protection of metals, why is it called sacrificial anode?

In the equipment made of metal BAI iron, iron is easy to form electrochemical corrosion. Iron as anode zhi in du formed battery is corroded, causing damage to the equipment. Dao so in the protection with a more active metal as the anode is corroded off, to protect the metal iron equipment, so that the service life of the equipment is extended. The metal used to protect the device is called a sacrificial anode, and a common sacrificial anode is magnesium.