An anode rod is a sacrificial corrosive rod inside of hot water heaters. This is a replacable part, even though most folks ignore the hot water heater until it starts leaking. Once that starts happening you have to replace the defective heater.
Inside the tank, water, especially hard water, corrodes the inside surface. Once a pinhole is rusted through the tank fails and water leaks out. The anode is designed to corrode before the tank rusts, since it's mostly made from iron. This protects the tank from failing. The anode rod is replaced by turning off the heater, closing the water infeed at the heater or for the whole house, opening up a hot water tap, then draining several gallons from the heater using the bottom hose fitting. Once that is done, there is a piece on the top that one unscrews to reveal a large metal corroded rod. Just pop a replacement in and turn the water back on.
There are solid rods and rods that fold so you can get them in taller tanks. Note that plastic tanks like the Marathon brand do not use anode rods.
Iron Noder 2017