So I understand that this corrodes to keep the hull of the boat from corrodong but why? And what causes the corrosion in the first place?
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Put two different metals in contact with each other via an electrolyte and you get what’s called a galvanic effect. That makes one metal slowly corrode.
Sea water is a pretty good electrolyte for this since it is electrically conductive to an extent.
Metals have different galvanic potential: https://www.mfcp.com/technical-info/galvanic-corrosion
If you put metals with similar galvanic potential together, they will corrode very slowly or not at all. The moment you have metals with different potentials, you get the corrosion.
The thing is that sometimes, you want the different metals because of their other properties. Steel is very good for structural purposes, but you may need copper for something else, and so on.
The anode is put there to have a metal with a galvanic potential that makes it corrode first. It will corrode preferentially over the other metals in the boat (or other device).
There’s a magnesium anode in the water heater in your home for that reason too. Magnesium is basically the first thing that will corrode, so it’s great as a sacrificial anode.
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