There are a lot of electrical systems on a boat, lights, instruments, engines, GPS and many more. If you go off shore there will come a time that some electrical device will not work and so it will be up to you to figure out why and then fix it. Your power comes from batteries, they have only so much stored electricity so they need to be charged, from your engine's alternator, shore power, solar, wind or a hydro-generator. Once charged the stored electricity goes thru wires to run your devices. That is the big picture. You need to learn about AC and DC power, volts and amps, inverters and chargers and much, much more.