If you have a tanks that provides your hot water, you will have waterheater anodes. The anode is an essential part of the water tanks, and it is also a part that tends to wear out first.
The anode is put into hot water tanks because being immersed in water is not good for metal - including the heating elements. Obviously if the elements become corroded or caked with deposits it will not work well. The job of the anode is to divert all this bad stuff from the elements, keeping them clean and doing their job. In other words, the anode corrodes so the heating elements do not have to.
Of course since it is designed to corrode, that is just what it does. How quickly this happens depends on your water (for instance if you have some device that softens you water you may find there is no corrosion problem), but in some cases it makes sense to replace it. The question is when is it worth it?
This question arises for two reasons. Firstly, the anode will cost you some money to buy. And secondly, they can be quite hard work to remove. What we need to decide is if the new anode will prolong the life of the unit. Remember that the heater itself can fail and that the typical life of a hot water tank is between ten and fifteen years. Added to this the new high efficiency tanks that will save money in the long run, and it is easy to see that in many circumstances replacing the anode is not cost effective. But if you have hard water and a new boiler, then the job may be one worth taking on.