Diodes are electrical one-way valves and are used in many applications in boat from large to small, but you may not recognise them at first. I showed you the diagram of the galvanic isolator elsewhere which comprises 4 diodes arranged like below, but how do you test one?
Most, even cheap digital mustimeters have a diode test position. You need to understand that the diode tester passes a small current (usually around 1mA) from the positive lead, through the diode you’re testing to the negative lead and you are then in fact measuring the forward voltage drop of the diode, which will be in the region of 0.5 to 0.7Volts. Most meters can in fact measure up to nearly three volts, so two diodes in series (like above) can be measured as good.
Where else are diodes found you might really need to know about? Alternators and DC power supplies.