What is cathodic protection at the expense of the anode?

Why is cathodic protection at the expense of the anode? Not the anode protection at the expense of the cathode?

Sacrifice anode cathodic protection law is, in fact, the positive protection law of the cathode due to the cathode and anode are electronic, oxidation reaction and the positive and Yin are electronic reduction reaction and in order not to let a metal erosion than his lively in metal ACTS as the cathode (anode) so is the sacrificial anode cathodic protection